These days, I looked through the Strategy Articles on the forum and realized that there is no guide to Medieval warfare. There is a lot on early strategy which is the most important, but many players lose focus in the Middle Ages (sometimes myself as well).
This is arguably the most dangerous era and most of my lost games could be pinpointed to poor play around this time in the game assuming I didn't make silly early mistakes. Production is relatively low still and slavery efficiency is low because cities are large, most forests are chopped, and drafting is still far on the tech line. Even worse, this seems to be the time that is most difficult to make a military push (more on this later) against the AI. AI's often have strong militaries (at least if you play on a level that is competitive for you) and on top of that Longbowmen are probably the strongest city defenders in the game relative to available attacking troops. Even when I play a military route (I play all my games on Monarch or Emperor), I often find myself below average in power at this stage and there is usually one AI or even two that is stronger than me and does not like me very much. So, on with the guide!!
Key Units:
Longbowmen (L-bows)
Crossbowmen (X-bows)
Macemen (maces)
Pikemen (pikes)
Knights
Trebuchets (trebs)
Other Units (Classical Era) that are still useful: War Elephants (ellies), Catapults (catas); both are enabled once Construction is reseached
Key Military Technologies
Feudalism - enables L-bows, enables Vassalage, requires Monarchy
Machinery - enables X-bows, enable maces (with CS), requires Metal Casting
Civil Service (CS) - enables maces (with Machinery), enables Bureaucracy, requires Code of Laws or Feudalism, requires Mathematics
Guilds - enables knights (also need Horseback Riding), requires Feudalism, requires Machinery
Engineering - enables pikes, enables trebs, enables castles, +1 road movement, requires Machinery, requires Construction
L-bows
These units are your all purpose city defenders. In other words, they can defend cities against anything except Knights very well. If your cities have walls and castles (at Engineering), your Longbowmen get even stronger and practically invincible (even Knights cannot work without ample siege support). However, you should only build significant amounts of these if you anticipate fighting to occur in your cities. Outside of cities and in city attack, these units are very poor (they die to Classical era units like Swordsmen and Horse Archers if not on defensive terrain). They have practically no offensive use. If you do expect to face an attack, research Feudalism ASAP. If facing massed Knights/Cuirassiers, switch to Pikemen unless you have no Iron or just want to be pillaged back to the stone age because L-bows can't do anything against those in the field.
Synergies:
1) Protective trait - +20 city defense and 1FS chance is a significant boost
2) Barracks - gives 3XP; you get 1 free promotion; most synergistic is +25% city defense or if defending hills, go after CI followed by the Guerilla line; if defending other tiles (like forest) against a particular type of unit, go for a specialized promotion like Shock
3) Totem Pole (Native America) - gives 3XP to Archery units; you get 1 free promotion; combined with Barracks and Protective, you can get CGI/CGII/CGIII and DI L-bows out of the gate
4) Walls, Castles, Culture, Hills for extra city defense
With even a few of the above synergies, your cities become very difficult to take down. Likewise do not attack Sitting Bull after he has Engineering unless you have plenty of Riflemen and Cannon. You have been warned.
X-bows
These are the ultimate anti-melee unit. Given that Macemen are some of the most abundant units, you may be glad you built a few of those. Don't overdo on these since they are pretty bad in attacking cities against an enemy that defends with L-bows. Don't rely on these for city defense either, they are not better than L-bows since get +50% city defense that is basically equivalent to X-bow +50% melee defense. On top of that, L-bows are better at defending against non-melee units. X-bows are good stack protectors in the Middle Ages and a few should be included to take care of coutner-attacking melee units. Even if you have maces, you don't want these getting injured since they are often your primary attackers. You need Iron to build X-bows.
Synergies:
1) Protective trait - +20 city defense and 1FS chance is a minor boost (only 1FS chance is important here)
2) Barracks - gives 3XP; you get 1 free promotion; most of the time, going after CI/Shock followed by more combat is best to increase the X-bow's strength against melee
3) Totem Pole (Native America) - gives 3XP to Archery units; you get 1 free promotion (see Barracks for synergistic promotions)
Chu-ko-nu
Think these units are just X-bows. Think again.. they are like X-bows, but have 1 extra inherent FS and do collateral damage!! That's not even it as they do +50% more collateral damage than trebs and +25% more than cats and can kill the units they are attacking. When playing China, get Machinery asap. The synergies are same as regular X-bows although Cover promotions may be a good option if CKN's used for city attack.
Nice Strategy:
Play as Qin, build the Oracle (research Iron Working in the meantime) and then take Metal Casting as free tech. Build/chop/whip a Forge in any city other than the one that built the Oracle and run an Engineer specialist. When it pops, bulb Machinery. Make sure you have researched Archery in the meantime. If you are not surrounded by violent neighbors, the second city that has the Forge can build the Pyramids to speed up the engineer. You can get a decent force (8-10) of CKN's by 500BC. Make sure your cities have Barracks, give your boys Cover and attack the enemy. Keep producing more CKN's. Once the first enemy is done, attack the next one. You have until ~500 AD until L-bows start popping up. Use the window effectively.
Note: don't try if surrounded by psycho AI's (Monty, Shaka come to mind), don't try if no other enemy on landmass, don't try if you have no source of Iron.
Later in the medieval age, CKN's are useful to use as siege weapons with their relatively high strength (and thus high survival) and large collateral damage. They also excel in field combat against anything other than Knights. Unlike normal X-bows, they should be used offensively.
Maces
These guys are your first medieval offensive units with a considerable strength of 8. You might think they are great anti-melee units with their +50% against melee, but they aren't. Enemy maces get +50% against you so it cancels out. X-bows are more efficient at dealing with enemy maces (cheaper + get slightly higher odds). Maces require Iron or Copper to build and knowledge of CS and Machinery. If you are first to get these, train a few and attack your nearest enemy who may still have archers on defense. Maces work well with City Raider (CR) promotions. You can give one or two CI followed by Shock particularly if you don't have Iron to build X-bows. Against L-bow defenders, maces won't cut it without siege support. Likewise against enemies who have X-bows - X-bows can actually kill you in the field too so make sure you have an escort if you know your enemies have X-bows. A L-bow (or even a mace) with CI/Cover will do ok. If facing Knights, get some pikes/ellies as escort.
Synergies:
1) Aggressive trait - free CI, open up level 2 promos like Shock and Cover
2) Barracks - gives 3XP; you get 1 free promotion; most of the time (see above), promote down the CR line
Berserker
These guys are great on water maps with no amphibious penalty (they attack coastal targets from ships just like land targets). They are not exactly bad on land either since Ragnar is Aggressive and they get an inherent +10% attack vs cities. With Barracks, you get +10% strength AND +30% attack vs. cities. Amphibious may also come in useful if you attack across rivers. Still, these units are meant to be used on water maps. It's worth beelining these on such maps and raiding enemy coastal cities. Even just going around razing cities will get you cash and seriously weaken your enemies who may still have Archers on defense.
Samurai
Since Toku is Aggressive, you are getting maces with 2 inherent FS, 1 FS chance (Drill I), and +10% strength (from CI). These units are significantly tougher than maces and thus can do much better in attacking L-bow defended cities. Give them Cover followed by the CR promotions. You should use siege to soften up the enemy, however. This unit requires Iron (cannot build with Copper) and it's worth beelining. Certainly a better unit than a Berserker on a land map although they are vulnerable to knights - FS don't help.
Pikes
These guys are your primary anti-mounted guys. Much like L-bows and X-bows, they are generally terrible city attackers (unless cities are guarded by mounted units). As city defenders, they do excel against Knights and can engage Knights in the field as well (remember that pikes are much cheaper than knights). If you are facing Knights, beeline Engineering and train a couple of these guys. You'll be glad you did. They are good to include in any stack as stack protectors. Optimal promotion line is the one that emphasizes the pike's strength - CI/CII/Formation/CIII... Pikes with Formation are even more cost-effective Knight killers.
Synergies:
1) Aggressive trait - free CI, open up level 2 promos like Shock and Cover
2) Barracks - gives 3XP; you get 1 free promotion; most of the time (see above), promote down the CR line
Landsknecht
There are often heated debate whether this unit is good. I am not quite sold on this unit. It is practically a hybrid between a X-bow and a pike (effective +50% vs. macemen and +100% vs. knights) and as such not a good city attacker. It is a good all purpose defender and can be an effective pillager, but it is very vulnerable to enemy X-bows. It should be escorted with a CI/Cover L-bow or something similar. It is generally good in field battles because of its high strength against melee and mounted as long as there are no X-bows. Make some of them anti-melee specialists (CI/Shock/CII...) and some of them anti-mounted specialists (CI/CII/Formation/CIII...). This units can save you some hammers when used as a stack defenders. Instead of having x X-bows + x pikes, have x landsknecht.
Knights
Build a lot of these fast units. Knights do exceptionally well in the field against maces, L-bows, X-bows and nothing before pikes can stop them. Against L-bows in cities, they do fairly well with high attack strength and immunity to first strikes. Once pikes show up, progress with just knights become difficult and ample siege support and anti-melee escort like maces or X-bows is a must. Knights don't receive defensive bonuses so they are not great when defending, but they are not horrible either. If defending your own cities, use your knights to attack the enemy stack - you can hurt them badly if they have no pikes since you can cause flanking damage and weaken their siege at relatively little cost. Knights benefit most from combat and after Engineering a Shock promotion is a good choice to help with pikes. Knights require Iron and Horses to build so make sure you have these resources. Research Guilds early if you plan to war in the Middle Ages.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Stable - gives 2XP; you get 1 free promotion; Ger (Mongols) - gives 4XP
Camel Archers
Certainly not one of the strongest UU's but definitely one of the most underrated. Stats wise, its just like a knight and generally has the same uses, but with 15% withdrawal chance. Your best bets for promotions are Flanking I and II to give your camel archers 45% withdrawal chance. This makes them one of the most cost-effective killers of L-bows in cities. You can even destroy pikes if you have numbers without too many casualties. Remember, units that retreat can fight another day. Camel archers in large numbers are very scary. On top of what I just mentioned, camel archers are resourceless!! In one of those games where you are missing Horses or Iron, you can still build them. Furthermore, your enemy cannot prevent you from building them by pillaging your resource.
Cataphracts
These guys are what I imagine when I think of heavy cavalry. They high strength makes them the ultimate field unit - it is on even ground even with pikes (its counter) while it own any other medieval unit pretty hard. Attacking cities with L-bows is much easier although catas do not have first strike immunity so Drill heavy L-bows (you'll proly only see those in MP) may be harder than usual. Overall, one of the strongest UU's in the game, no doubt about it. Beeline Guilds when you play the Byzantines. Recommended promotions and resource requirements are same as for regular knights.
Catapults
These guys become available at Construction and are a siege weapon of choice in the early medieval age. They are excellent in field combat (better than trebs in fact), but die in large numbers facing fortified L-bows in cities. If your enemies have castles (enabled by Engineering), forget about using cats - not cost efficient at all. Get Engineering yourself and get trebs. Generally give catapults CR (for attacking cities). If you have a few for field combat or already have trebs, give them Barrage. Escort these units with your primary attackers like knights and other specialized defenders like pikes and X-bows.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Citadel (Spanish Castle) - gives 5XP to siege units, expires at Economics (avoid this tech when playing as Spain)
Trebs
Your best bet for killing those pesky L-bows hiding in well fortified cities. With +100% city attack, trebs have a much much higher survival rate than cats although they do deal slightly less collateral damage. Once you get them, don't go back to using cats for attacking cities. Trebs are also better at bombarding city defenses. Escort these units with your primary attackers like knights and other specialized defenders like pikes and X-bows.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Citadel (Spanish Castle) - gives 5XP to siege units, expires at Economics (avoid this tech when playing as Spain)
Phants
Also available at Construction, these guys are a force in the Classical Era although they become weaker in the medieval era. Their anti-mounted bonus gives you ability to counted mounted units before Engineering. Even once you get pikes, you may still want to have these guys. They are slightly better against Knights and can be utilized offensively to clean up while pikes are too weak for any offensive use. Beware of enemy pikes though - they eat phants for breakfast. Phants require Ivory to build, which is a relatively rare resource so you won't build these in the majority of your games. Give these guys combat and formation promotion (once you get CII) to enhance their ability against mounted units further.
Ballista Phants
Have a unique ability of being able to selectively kill mounted units when attacking. These means you can even attack enemy stack with pikes to kill their mounted units. This is nice ability because you get a change to remove the most dangerous offensive units in an enemy stack in a cost-effective manner.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Stable - gives 2XP; you get 1 free promotion; Ger (Mongols) - gives 4XP
General Military Synergies:
1) Settling GG's - gives units in that city +2XP
2) Attaching GG's to a unit - distributes 20XP even into all units in a stack (can give a single unit 20XP to make a supermedic or other specialized unit)
3) Theocracy (Theology) - gives units in all cities +2XP
4) Vassalage (Feudalism) - gives units in all cities +2XP, gives free unit support (# depends on map size)
5) Using spies to cause revolt - can reduce city defense completely for one turn. Works well with fast units like knights where speed is important. It may not be easy to get a lot of espionage points especially against a large enemy empire.
This is arguably the most dangerous era and most of my lost games could be pinpointed to poor play around this time in the game assuming I didn't make silly early mistakes. Production is relatively low still and slavery efficiency is low because cities are large, most forests are chopped, and drafting is still far on the tech line. Even worse, this seems to be the time that is most difficult to make a military push (more on this later) against the AI. AI's often have strong militaries (at least if you play on a level that is competitive for you) and on top of that Longbowmen are probably the strongest city defenders in the game relative to available attacking troops. Even when I play a military route (I play all my games on Monarch or Emperor), I often find myself below average in power at this stage and there is usually one AI or even two that is stronger than me and does not like me very much. So, on with the guide!!
Key Units:
Longbowmen (L-bows)
Crossbowmen (X-bows)
Macemen (maces)
Pikemen (pikes)
Knights
Trebuchets (trebs)
Other Units (Classical Era) that are still useful: War Elephants (ellies), Catapults (catas); both are enabled once Construction is reseached
Key Military Technologies
Feudalism - enables L-bows, enables Vassalage, requires Monarchy
Machinery - enables X-bows, enable maces (with CS), requires Metal Casting
Civil Service (CS) - enables maces (with Machinery), enables Bureaucracy, requires Code of Laws or Feudalism, requires Mathematics
Guilds - enables knights (also need Horseback Riding), requires Feudalism, requires Machinery
Engineering - enables pikes, enables trebs, enables castles, +1 road movement, requires Machinery, requires Construction
L-bows
These units are your all purpose city defenders. In other words, they can defend cities against anything except Knights very well. If your cities have walls and castles (at Engineering), your Longbowmen get even stronger and practically invincible (even Knights cannot work without ample siege support). However, you should only build significant amounts of these if you anticipate fighting to occur in your cities. Outside of cities and in city attack, these units are very poor (they die to Classical era units like Swordsmen and Horse Archers if not on defensive terrain). They have practically no offensive use. If you do expect to face an attack, research Feudalism ASAP. If facing massed Knights/Cuirassiers, switch to Pikemen unless you have no Iron or just want to be pillaged back to the stone age because L-bows can't do anything against those in the field.
Synergies:
1) Protective trait - +20 city defense and 1FS chance is a significant boost
2) Barracks - gives 3XP; you get 1 free promotion; most synergistic is +25% city defense or if defending hills, go after CI followed by the Guerilla line; if defending other tiles (like forest) against a particular type of unit, go for a specialized promotion like Shock
3) Totem Pole (Native America) - gives 3XP to Archery units; you get 1 free promotion; combined with Barracks and Protective, you can get CGI/CGII/CGIII and DI L-bows out of the gate
4) Walls, Castles, Culture, Hills for extra city defense
With even a few of the above synergies, your cities become very difficult to take down. Likewise do not attack Sitting Bull after he has Engineering unless you have plenty of Riflemen and Cannon. You have been warned.
X-bows
These are the ultimate anti-melee unit. Given that Macemen are some of the most abundant units, you may be glad you built a few of those. Don't overdo on these since they are pretty bad in attacking cities against an enemy that defends with L-bows. Don't rely on these for city defense either, they are not better than L-bows since get +50% city defense that is basically equivalent to X-bow +50% melee defense. On top of that, L-bows are better at defending against non-melee units. X-bows are good stack protectors in the Middle Ages and a few should be included to take care of coutner-attacking melee units. Even if you have maces, you don't want these getting injured since they are often your primary attackers. You need Iron to build X-bows.
Synergies:
1) Protective trait - +20 city defense and 1FS chance is a minor boost (only 1FS chance is important here)
2) Barracks - gives 3XP; you get 1 free promotion; most of the time, going after CI/Shock followed by more combat is best to increase the X-bow's strength against melee
3) Totem Pole (Native America) - gives 3XP to Archery units; you get 1 free promotion (see Barracks for synergistic promotions)
Chu-ko-nu
Think these units are just X-bows. Think again.. they are like X-bows, but have 1 extra inherent FS and do collateral damage!! That's not even it as they do +50% more collateral damage than trebs and +25% more than cats and can kill the units they are attacking. When playing China, get Machinery asap. The synergies are same as regular X-bows although Cover promotions may be a good option if CKN's used for city attack.
Nice Strategy:
Play as Qin, build the Oracle (research Iron Working in the meantime) and then take Metal Casting as free tech. Build/chop/whip a Forge in any city other than the one that built the Oracle and run an Engineer specialist. When it pops, bulb Machinery. Make sure you have researched Archery in the meantime. If you are not surrounded by violent neighbors, the second city that has the Forge can build the Pyramids to speed up the engineer. You can get a decent force (8-10) of CKN's by 500BC. Make sure your cities have Barracks, give your boys Cover and attack the enemy. Keep producing more CKN's. Once the first enemy is done, attack the next one. You have until ~500 AD until L-bows start popping up. Use the window effectively.
Note: don't try if surrounded by psycho AI's (Monty, Shaka come to mind), don't try if no other enemy on landmass, don't try if you have no source of Iron.
Later in the medieval age, CKN's are useful to use as siege weapons with their relatively high strength (and thus high survival) and large collateral damage. They also excel in field combat against anything other than Knights. Unlike normal X-bows, they should be used offensively.
Maces
These guys are your first medieval offensive units with a considerable strength of 8. You might think they are great anti-melee units with their +50% against melee, but they aren't. Enemy maces get +50% against you so it cancels out. X-bows are more efficient at dealing with enemy maces (cheaper + get slightly higher odds). Maces require Iron or Copper to build and knowledge of CS and Machinery. If you are first to get these, train a few and attack your nearest enemy who may still have archers on defense. Maces work well with City Raider (CR) promotions. You can give one or two CI followed by Shock particularly if you don't have Iron to build X-bows. Against L-bow defenders, maces won't cut it without siege support. Likewise against enemies who have X-bows - X-bows can actually kill you in the field too so make sure you have an escort if you know your enemies have X-bows. A L-bow (or even a mace) with CI/Cover will do ok. If facing Knights, get some pikes/ellies as escort.
Synergies:
1) Aggressive trait - free CI, open up level 2 promos like Shock and Cover
2) Barracks - gives 3XP; you get 1 free promotion; most of the time (see above), promote down the CR line
Berserker
These guys are great on water maps with no amphibious penalty (they attack coastal targets from ships just like land targets). They are not exactly bad on land either since Ragnar is Aggressive and they get an inherent +10% attack vs cities. With Barracks, you get +10% strength AND +30% attack vs. cities. Amphibious may also come in useful if you attack across rivers. Still, these units are meant to be used on water maps. It's worth beelining these on such maps and raiding enemy coastal cities. Even just going around razing cities will get you cash and seriously weaken your enemies who may still have Archers on defense.
Samurai
Since Toku is Aggressive, you are getting maces with 2 inherent FS, 1 FS chance (Drill I), and +10% strength (from CI). These units are significantly tougher than maces and thus can do much better in attacking L-bow defended cities. Give them Cover followed by the CR promotions. You should use siege to soften up the enemy, however. This unit requires Iron (cannot build with Copper) and it's worth beelining. Certainly a better unit than a Berserker on a land map although they are vulnerable to knights - FS don't help.
Pikes
These guys are your primary anti-mounted guys. Much like L-bows and X-bows, they are generally terrible city attackers (unless cities are guarded by mounted units). As city defenders, they do excel against Knights and can engage Knights in the field as well (remember that pikes are much cheaper than knights). If you are facing Knights, beeline Engineering and train a couple of these guys. You'll be glad you did. They are good to include in any stack as stack protectors. Optimal promotion line is the one that emphasizes the pike's strength - CI/CII/Formation/CIII... Pikes with Formation are even more cost-effective Knight killers.
Synergies:
1) Aggressive trait - free CI, open up level 2 promos like Shock and Cover
2) Barracks - gives 3XP; you get 1 free promotion; most of the time (see above), promote down the CR line
Landsknecht
There are often heated debate whether this unit is good. I am not quite sold on this unit. It is practically a hybrid between a X-bow and a pike (effective +50% vs. macemen and +100% vs. knights) and as such not a good city attacker. It is a good all purpose defender and can be an effective pillager, but it is very vulnerable to enemy X-bows. It should be escorted with a CI/Cover L-bow or something similar. It is generally good in field battles because of its high strength against melee and mounted as long as there are no X-bows. Make some of them anti-melee specialists (CI/Shock/CII...) and some of them anti-mounted specialists (CI/CII/Formation/CIII...). This units can save you some hammers when used as a stack defenders. Instead of having x X-bows + x pikes, have x landsknecht.
Knights
Build a lot of these fast units. Knights do exceptionally well in the field against maces, L-bows, X-bows and nothing before pikes can stop them. Against L-bows in cities, they do fairly well with high attack strength and immunity to first strikes. Once pikes show up, progress with just knights become difficult and ample siege support and anti-melee escort like maces or X-bows is a must. Knights don't receive defensive bonuses so they are not great when defending, but they are not horrible either. If defending your own cities, use your knights to attack the enemy stack - you can hurt them badly if they have no pikes since you can cause flanking damage and weaken their siege at relatively little cost. Knights benefit most from combat and after Engineering a Shock promotion is a good choice to help with pikes. Knights require Iron and Horses to build so make sure you have these resources. Research Guilds early if you plan to war in the Middle Ages.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Stable - gives 2XP; you get 1 free promotion; Ger (Mongols) - gives 4XP
Camel Archers
Certainly not one of the strongest UU's but definitely one of the most underrated. Stats wise, its just like a knight and generally has the same uses, but with 15% withdrawal chance. Your best bets for promotions are Flanking I and II to give your camel archers 45% withdrawal chance. This makes them one of the most cost-effective killers of L-bows in cities. You can even destroy pikes if you have numbers without too many casualties. Remember, units that retreat can fight another day. Camel archers in large numbers are very scary. On top of what I just mentioned, camel archers are resourceless!! In one of those games where you are missing Horses or Iron, you can still build them. Furthermore, your enemy cannot prevent you from building them by pillaging your resource.
Cataphracts
These guys are what I imagine when I think of heavy cavalry. They high strength makes them the ultimate field unit - it is on even ground even with pikes (its counter) while it own any other medieval unit pretty hard. Attacking cities with L-bows is much easier although catas do not have first strike immunity so Drill heavy L-bows (you'll proly only see those in MP) may be harder than usual. Overall, one of the strongest UU's in the game, no doubt about it. Beeline Guilds when you play the Byzantines. Recommended promotions and resource requirements are same as for regular knights.
Catapults
These guys become available at Construction and are a siege weapon of choice in the early medieval age. They are excellent in field combat (better than trebs in fact), but die in large numbers facing fortified L-bows in cities. If your enemies have castles (enabled by Engineering), forget about using cats - not cost efficient at all. Get Engineering yourself and get trebs. Generally give catapults CR (for attacking cities). If you have a few for field combat or already have trebs, give them Barrage. Escort these units with your primary attackers like knights and other specialized defenders like pikes and X-bows.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Citadel (Spanish Castle) - gives 5XP to siege units, expires at Economics (avoid this tech when playing as Spain)
Trebs
Your best bet for killing those pesky L-bows hiding in well fortified cities. With +100% city attack, trebs have a much much higher survival rate than cats although they do deal slightly less collateral damage. Once you get them, don't go back to using cats for attacking cities. Trebs are also better at bombarding city defenses. Escort these units with your primary attackers like knights and other specialized defenders like pikes and X-bows.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Citadel (Spanish Castle) - gives 5XP to siege units, expires at Economics (avoid this tech when playing as Spain)
Phants
Also available at Construction, these guys are a force in the Classical Era although they become weaker in the medieval era. Their anti-mounted bonus gives you ability to counted mounted units before Engineering. Even once you get pikes, you may still want to have these guys. They are slightly better against Knights and can be utilized offensively to clean up while pikes are too weak for any offensive use. Beware of enemy pikes though - they eat phants for breakfast. Phants require Ivory to build, which is a relatively rare resource so you won't build these in the majority of your games. Give these guys combat and formation promotion (once you get CII) to enhance their ability against mounted units further.
Ballista Phants
Have a unique ability of being able to selectively kill mounted units when attacking. These means you can even attack enemy stack with pikes to kill their mounted units. This is nice ability because you get a change to remove the most dangerous offensive units in an enemy stack in a cost-effective manner.
Synergies:
1) Barracks - gives 3XP; you get 1 free promotion
2) Stable - gives 2XP; you get 1 free promotion; Ger (Mongols) - gives 4XP
General Military Synergies:
1) Settling GG's - gives units in that city +2XP
2) Attaching GG's to a unit - distributes 20XP even into all units in a stack (can give a single unit 20XP to make a supermedic or other specialized unit)
3) Theocracy (Theology) - gives units in all cities +2XP
4) Vassalage (Feudalism) - gives units in all cities +2XP, gives free unit support (# depends on map size)
5) Using spies to cause revolt - can reduce city defense completely for one turn. Works well with fast units like knights where speed is important. It may not be easy to get a lot of espionage points especially against a large enemy empire.