Here's another Medieval II Total War modification.
I have looked into every building and unit in the game and changed many prerequisates as well as added eras for these buildings/units.
There are now 8 eras in the game where buildings or units may appear and correspond to historical events within the game and the dates that these events happened in real life.
1) First Windmill 1130: mounted crossbows
2) First European Paper 1152: Teutonic and Santiago Knights
3) Mongols Invasion 1208-1224: new knights, early polearms
4) Gundpowder Discovered 1240-1250: some new buildings - however does not allow immediate building of gunpowder units (too early)
5) First Eyeglasses 1286: longbows, late polearms, first cannons, early pikes
6) Timurids Invasion 1368-1384: Plate armor, better pikes, more cannons
7) First Piano 1400: Gothic armor, arquebus, hand guns
8) World is Round (true time) 1446-1453: Carrack, muskets
And of course, era 0, which are the units available from the start.
Units can be recruited from buildings that match their unit type, relative strength, armor level, training, skill, ability, and specialization.
Recruitment pools are also slightly smaller and slower especially for artillery and mounted units, which, in theory, should help the AI recruit better armies.
-balance of realism and gameplay
-completely new building tech tree
-units correspond to realistic eras
-units are built in buildings that correspond to their armor, training, skill, and specialization
-more building dependencies (prerequesites making buildings important, especially smiths)
-faction specific buildings tweaked
-Horsebreeder's Guild now built in castles (by looking at the triggers, I'm convinced that this guild was meant to be in castles)
-added in the "realistic" missing units for balance
-reduced recruitment pools for better AI armies