Can't place defense blueprints on random tiles.

In the Under Games campaign, there were several levels where random tiles would not allow defense blueprints. Some tiles would allow traps such as the Bombard but then wouldn't allow a rampart to be placed over a completed Bombard. I may be wrong, but I seem to remember all of these maps having Banshee shrines on them. Maybe banshees having blown up in the area glitched out some tiles?