When we think of the Middle Ages, almost everyone living in the western hemisphere thinks of knights and castles. It's just too significant of a unit to do away with.
That said, the threads about the medieval UUs- those units themselves are weak. I don't actually think the knight unit is too strong
relative to how the game appears to have been intended to be balanced.
The heavy cav line pre-tanks has two units: chariots and knights. Let's look at chariots first.
-Strength 28
-move 2 or 3
-cost 65.
There are two contemporary units in the ancient era:
Archers (ranged attack 25) and Spears (strength 25.) Both also cost 65. I'm ignoring warriors because they are a special case.
So the heavy cav has the attribute of having +3 combat strength and an extra movement over his unmounted peers.
Now let's go to the middle ages.
Knight: 48, cost 180
Pikes: 41, cost 200
Xbows: 40, cost 180.
Huge amounts have been written on the fact that pikes simply cost way too much. Let's ignore that for a second. Do you see how knights have gained 20 strength- 10 per era (ancient->classical->medieval)? This is a feature of almost every unit line, even many siege units.
See here where I bring it up.
In fact, the pikeman for whatever reason does not follow this trend, and has 41 strength when it looks they should have 45. Note that Pike and shot have 55- this is the only one era upgrade of +15 excluding warriors.
Now, if pikes had 45 strength and cost 180, then we would in a very different ballgame. We would have almost identical mechanics to the ancient era, with H Cav getting +3 strength and a movement advantage over the footmen.
Knights have the same power curve that melee and ranged units do- about +10 per era** and chariots aren't decried as OP. This leads me to think that for whatever reason, the incredible decision to make pikes ineffective at stopping knights is making them look much better. We also have no medieval melee base unit, making things even worse. But look at the samurai- it has 45 strength and costs 180. The downside is you cannot upgrade swords into them, but samurai units themselves are the only military tactics unit that is actually not harshly spoken about. Go play Japan and build them. They do pretty well for themselves- now imagine if we had this for the middle ages UUs:
Military tactics unique units: 45 strength, 180 cost
Samurai bonus: no penalty for being injured (I think this penalty goes up to 10ish?)
Khevsur bonus: +7 on hills
Berserker bonus: +7 on attack, -7 on defense
If you then let people upgrade swords into those units, you would have mostly phenomenal balance (it would help if military tactics wasn't a leaf tech.). Knights would still be at the top of the food chain, but would need to be wary of pikes,
the unit dedicated to countering them. But crossbows and melee units would be cut down on the field. Think of when you've fought knights with muskets- they aren't dying like flies, they hold up decently- that's how pikes would do vs knights.
**I think a couple early units- archers & warriors in particular- have special balancing because they either have to fill in for a significant amount of time (archers have to fight swords and horses too!) or because you get a free one and can build them on turn 1 (warriors.) Knowing how crazy good ranged units were in Civ5 I think it's fair if they had ranged units be a -5 relative to their contemporaries. The transition to modern has a few units that jump a little- infantry, AT crew, and tanks- which are almost undoubtedly due to a desire to have round numbers and the fact that the large gaps in the unit tree dicatte you can't have upgrades be too good. Armored units were initially released having +10 strength over the boots on the ground, but they buffed mech inf to be 85.
