The building with the "prison" logo on the outside, located towards the southeastern area of Brest.
Several rooms of various sizes are linked together by hallways. The monster count varies behind the doors. Quite a few secret doorways as well.
You can get by with level 12 or so, but some of the areas require level 20 or higher for all classes.
Titans, Rust Monsters, Orcs, Knolls, Kobolds, Blobs, Skulls, Wraiths, Zombies, Bats, Hill Giants, Wyverns, Ghosts, Ants, Violent Fungi, Snakes, Bees, Behemoths, Acid Spheres, Demon Lords, Red Dragons, Electric Dragons, Chinese Dragons, Demi Liches, Brown Wizards