In the original Dungeon Master, the scare was the enemy movement sound getting ever louder before you could actually see them 
It was real time, but a bit slow, like you already have there. Not knowing what is coming. But brewing the last potions and typing in the spell runes before the movement sound turned into monsters just appearing around the corner was quite the deal…
It was not really about the monsters being scary. One monster was a tree with butt holes, A TREE! Not extremely scary.
In Skullstone every monster have its own sounds for movement and other actions. It works well, you can hear a monster walking behind the doors or somewhere behind the wall.
I’m thinking about another feature - some mobs would affect the lights, especially the torchlights by changing its power and/or color. So when you see that all lights dims you’ll know that something is waiting for you.
Good idea. Like in horror movies, something very sinister is coming your way and the pure evil of it disturbs your torch like a sudden draft of air.