Thanks a lot!
Not being able to build on top of the mines is intentional, but now I find it weird myself.
I agree that waves are too punishing for the new players and the building system might need a rework.
As for the sprites not updating after building below, I got out of time to fix it, so here we are :D
We also considered destroying or dropping down existing buildings when the tile below got destroyed, but I felt that it would raise the difficulty too much for the jam game, and I had not enough time to change this behaviour at that moment :/