Had the same issue of not noticing the "door" opening and getting confused when I had 6/5 cheese and not knowing what to do.
Possible solutions here would be one or both of:
- Not spawning any more cheese when requirement is met.
- Put an arrow on the exit tile facing the direction of exit, or a staircase.
Also... you need to play test level 3: blast radius increase plus more bombs isn't too awful, but couple that with dying due to bomb blasting "your previous" location (meaning mouse is already dead whenever it passes the radius of a bomb that will explode 1 tick later) is just unfair.