When you capture a city through conquest, there is a percent chance that some buildings are destroyed. However even if you gain control of a city through trade or through a peace deal, some buildings still disappear: 1. It looks like defensive buildings (walls, castle) automatically go away even if the city is peacefully taken. 2. Pre-BNW, all cultural buildings were lost when a city changed owners. Post-BNW, most cultural buildings (with great work slots) stay intact though they can still sometimes get destroyed if you conquered the city. 3. I have noticed that some Unique Buildings get destroyed. Right now the popular consensus about UB's is that: If your civilization has a UB and you capture a city with the normal building, it automatically becomes the UB. If a normal civilization captures a city with a UB then it gets converted into a normal version of that building. Here is the thing: I am currently playing a game with a minor custom mod. Made it myself. It has very minor changes which I do not think should be significantly impacting the way the game works. In particular, I made a custom civilization that has two unique buildings (which already exist) - the Stele and Royal Library. So the thing I am noticing is that whenever I take control of a city with a Monument or Library it is converting to a Stele or Royal Library (which is expected). However, whenever my city (with Stele or Royal Library) becomes owned by another civ (including as part of a peace deal), the Stele or Royal Library is instantly destroyed (not converted into a normal version). Assyria happens to be in this game, and even though we both have Royal Libraries, they still get obliterated if control of a city changes. And it also applies when my custom civ is not involved. (Carthage got Ninevah in a peace deal. The Royal Library got obliterated, but the city retained its university but no library). On a different note, I did see that in Baghdad (originally with Arabia's Bazaar UB) the Bazaar did successfully convert into a normal Market. Which makes me wonder if it's something special with the Stele and Royal Library. Or my mod somehow messed things up (even though it is fairly simple and I don't think it should). So anyone else have experiences with UB's disappearing (and not getting a normal replacement) when a city changes control?