Only the empty slots.
A building on filled slot 0 remains there, and then
a building on filled slot 1 stays on 1, and so on, until an empty slot is found.
It can lead to a stale situation: If noone builds or burns, the same tiles remain forever. That is why I came up with Rampant Dragon burning separate territory building
. It happened to us and we are seasoned euro-style high-optimization players. Noone built or burnt. There were only "Separate Territory points" 2VP / territory buildings, too low reward on these.