Welcome to one of the big problems with Dungeon Keeper. Once a wall is knocked down, it's knocked down. If you open yourself up to a cavern full of guys you can't handle, there isn't much you can do, especially early in the game. There are things you can do to slow them down or thin their numbers but you can't actually lock them out
Spoiler
excluding certain unscrupulous methods involving temple and guard post tiles
.
The question is how do you go about mapping this while remaining both fair and interesting? Should the player always know there is going to be a fight? Shouldn't the player be wary about where they're digging in the first place? Even if you show the player a hero gate, how do they know what triggers it or how strong the attacking party will be? Dungeon Keeper is a great game but it's pumped full of bad design decisions.