One-way room layout

The pond in the ettin desert can be used as a one-way ledge, in spite of how low the surface of the pool is compared to the adjacent slope.

By changing the permeability of the doors lining the surface of the pond (as pictured above), creatures are still able to enter and exit the pond as if the doors had a permeability of 100.

But, if you were to drop the creature onto the pond...

They will walk along its surface. When the creature walks out of the pond this way, they will drop onto the slope and cannot walk back onto the surface of pool, walking through it and submerging themselves as shown in the earlier screenshots.

This is a very unusual oddity which you can try out for yourself. I think this happens because the floor doors are only checked for collision when they are being walked on and instead the open door leading into those rooms is checked for collision, and having a permeability of 100 it allows the creature to enter, even though the door is very small! This has the potential to be used for intentional one-way ledges in map design.