Linking buildings to the city type.

Gentlemen, I've encountered a problem... I decided to create my own map, and I kind of cobbled it together "from what I had," but I've run into an issue. There are 4 castles, and I've set it to random (so that you can choose a race in the game itself, but this doesn't affect the question), and each castle has buildings of level 1 and 2, where additional units can be recruited. So, how do I link the buildings to the castles so that when a castle is chosen, the appropriate building is available (in other words, I'm playing as Dwarves, but there's a Necropolis on the territory)?