How does the following look? The best part (I think) is that you do not need to add any new buildings to the game, and the maintenance costs of the defensive structures will finally be justified.
Mechanics:
When attacked, the healthiest unit defends.
Stacking penalty: Units take collateral damage (think how arty from Civ 4 worked, except its now applies to all attacks).
Most Civilizations
City : Can garrison 1 unit of any type.
City + City Walls : Can garrison 2 units. Limit: Cannot have more than 1 (Archer + Artillery)
City + City Walls + Castle : Can garrison 3 units. Limit: Cannot have more than 1 Artillery. Cannot have more than 2 (Archer + Artillery).
City + City Walls + Castle + Military Base: Can garrison 4 units. Limit: Cannot have more than 2 artillery. Cannot have more than 3 (Archer + Artillery).
Babylon
City : Can garrison 1 unit of any type.
City + Walls of Babylon: Can garrison 2 units. Limit: Cannot have more than 1 Artillery.
City + Walls of Babylon + Castle : Can garrison 3 units. Limit: Cannot have more than 1 Artillery.
City + Walls of Babylon + Castle + Military Base: Can garrison 4 units. Limit: Cannot have more than 2 Artillery.
India
City : Can garrison 1 unit of any type.
City + City Walls : Can garrison 2 units. Limit: Cannot have more than 1 (Archer + Artillery)
City + City Walls + Mughal Fort: Can garrison 3 units. Limit: Cannot have more than 2 Artillery. Cannot have more than 2 (Archer + Artillery).
City + City Walls + Mughal Fort + Military Base: Can garrison 4 units. Limit: Cannot have more than 3 Artillery. Cannot have more than 3 (Archer + Artillery).
Basically, it boils down to this:
- Each defensive structure increases Garrison size by 1
- Most civilizations can only garrison (size-1) range units.
- Cities can only field 1 artillery unit for defense until they build Military Base which lets them field 2.
- Babylons can field as many archers as garrison size (Walls of Babylon benefit), but are restricted to a maximum of 2 artillery units.
- Indians can field an extra artillery unit thanks to the Mughal Fort.
The Mughal fort should completely lose its unjustified and flavour-breaking "Gives gold after discovery of Flight" :: How the hell does that even work IRL?
Bablylonian walls could lose the 50% bonus to defense, though this is up to debate.
Chinese CKN and British Lbowmen might need a penalty in this setup. (CKN loses 1 attack, Lbow loses 1 range?)
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Explaining flavour:
- An undefended city can have any sort of unit in it.
- A city with city walls can have a melee garrison inside the city (which can venture out to attack from the city gates (think siege warfare), while having an archer garrison manning the parapets of the wall (Or Roman Scorpions mounted on the fort instead of archers, for instance).
- A city with a castle can have an archery garrison manning the parapets and another inside the towers - while having a melee unit for sallying forth! Alternatively, imagine cannon on the parapets (google images: Mughal Fort Cannon) with an archery garrison in the towers, and the obligatory melee.
- A city with the military base gets a chance to install another firebase for artillery, with combined arms and all that jazz.