So this the prison level of Harstaag Fort. There are cells, both barred and with optional iron doors, interrogation rooms, isolation cells, and guard-rooms.
However, since the fort was abandoned, salt water broke through the wall in the south west room and began to eat away at the floor. Following the path of least resistance, the water found a way down through a sinkhole it widened underneath the balcony. However, that whole lower level is completely flooded and the current in the deeper sections are strong.
It’s on this level that the corrupted elementals have made their home. One of the most fearsome creatures found here is the rare Salt-water Shambling Mound.