Wow! I can't believe this thread is still going after all these months!
I don't know if it's already been posted because this is my first time here, have you guys ever noticed that when you take over cities, that most city improvements just aren't there. They may have 1 or 2 or maybe even 3, but rarely. How can their city be size 12 without an Aqueduct?
There is an exact pattern to this. you'll spot it easily now: When a city is lost, the game decides to remove some improvements.... the way it does this is by hardcoding all possible improvements into two different groups, lets say Group 1 and Group 2. The game decides, at the moment of capture, on one group or the other. All the improvements in that category are then removed. There is no overlap, and the groups never change. If a city is conquered twice in a row, it is possible for it to lose all improvements. In practice, this rarely happens because of certain game biases which make one group or the other more likely to be chosen.
Take a look when you capture cities, and note the improvements. I'll start you with a hint. Aquaduct and Sewer are in the same group.... in a city with both these improvements, you'll always have both or none after capture.
About the Aquaduct question. For any city sized 8 to 127, you need an aquaduct for it to grow. For any city from size 12 to 127, you need both the Aquaduct and the Sewer for it to grow.
If you lose these, population is not lost.... it just prevents growth. If your city is fully grown, and you definately know you will never need it to grow larger in a game (say you have a size 33 city), then you can sell off both for 200 gold, and save 4 gold per turn. Once a city is done growing, Aquaducts and Sewers are not needed!
PS, The AI must abide by these same growth rules... it does not cheat on this.