What brozee is describing for example is if a 100 land 10 castle fully build out kingdom is hit they lose 10 and 1. So they should have 1 castle and less than 900 buildings. The hit might leave them with 905 buildings.
The game guide talks about how 1 castle can only control 45 land and having more than 450 buildings could be possible if an attack is made and non built land is taken.
The quickest way I know you can end up overbuilt is if you make a hit, destroy buildings, set buildings to build, get hit and lose the land you gained but already had buildings scheduled.