I started the game, played for like 2 or 3 turns then I noticed that my cities (not all of them) had a sudden 12.5% pop growth. I studied the situation for a while and I saw this.
I think it might be a bug because it suddenly showed up , never had that building before in any city. And it's also for castles and fortresses too.
Milan , Genova Pisa and Firenze have it , but Bologna doesn't. Also in Iberia Cordoba does not have it.