Pictured here is a section of wall crenellations. This is the first section on the right when you exit the barracks for the first time, but all similar wall sections on this level have the same bug. When rats spawn in during hordes and ambushes they climb over the edge of the wall, the rats then drop down into the ground about 4-5 feet, they are not visible and cannot be attacked. The moment they start to move they suddenly pop out of the ground again. Or if a player happens to be standing nearby they can attack them from inside the ground.
I think this is similar to the bug with spawn points on Bell where rats crawl out of medium height house windows and then drop through the ground in the same way.