Hole in the wall are devasting.
If you are a good player or a new one, quite often you have a gap between two building who make your defenses very wick. It’s very hard in sloping areas to see if there is a gap.
It could be wonderful to display in red all spaces of one square between two building or tree, gold pit… when you are placing a building.
It could be also in a mob but in the game, there is an option who display the grid.
I suggest to add “keep inclosed” notification which was in stronghold crusader game