Warden:
Hmm...
Since the dungeon was abandoned for so long, the prisoners -- monsters all -- eventually formed an independent society.
This basement camp features a very complicated network of secret passageways, which the monsters now use to move at will throughout this dungeon and even to areas of the surface world they inhabit.
They rule the underground world, which even has its own twisted class structure. Apparently higher-level monsters inhabit the dungeons beneath castles while and relatively low-level creatures live beneath fortresses.
"That certainly explains the endless stream of monsters in the basement prison despite years of bloody suppression..."