In short, there's 4 categories:
Defensive, Offensive, Harassment and Support
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Every "*" indicates a change of Era.
The defensives are: Spearmen,* Pikemen, Musketeers, * Riflemen, Infantry, * Mech Inf.
The offensives are: Archers, Swordsmen, * Med. Infantry, Longbowmen, *Guerilla, Marines
Harassment: Horsemen,* Knights, Cavalry, * Tanks, * Modern Armour. Also includes every marine vessel with an attack value.
Support: Worker, Catapult, * Trebuchet, Cannon, * Artillery, Fighter, Bomber, * Jet Fighter, Stealth Fighter, Stealth Bomber, Cruise missile, Tactical Nuke, Icbm, Radar Artillery. Also includes every naval unit with either Bombard capacity or Transport capacity.
The Warrior can be either defensive, offensive or harassment, but certainly not the preferred unit for it.
For the UU's, just check whatever unit it replaces and put in the corresponding category. Some UU's gain an extra movepoint, placing them from their original category into the Harassment category (for example: Jaguar Warrior, Impi, Gallic Swordsmen).
The Ancient Cavalry (produced by Statue of Zeus) is a Harassment unit, while the Crusader (Knights Templar) is an offensive unit.
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Defensive units are used to protect your cities, points of interest and your invading forces.
Offensive units are to be used solely for kicking your enemies bottom... I mean, rightfully annexing cities that are meant to be yours in the first place
Harrasment units have different uses, and they're good at all of it. You can use them offensively, you can use them to hunt down stray enemy units, you can use them for scouting or for pillaging your enemy's lands. The only thing they're not very good at, is defense. Naval harassment units can only be used for killing enemy vessels.
Support units are only used to help your invading forces, by either maiming (and occasionally killing) your enemy's garrisoned units, or by transporting much needed troops into enemy territory. While the worker is no true military unit, the roads he can build aid you in cutting down the time you need for reaching your goal. This certainly is the case for wheeled units, whom are restricted in there movement to certain terrains unless their is a road they can use.
While I guess you probably knew the uses for the different categories, I had to include it for the sake of completness.
I hope this helps you in your conquest to victory
