The white church in the middle of the Ruins.
The map consists of large rooms with some pretty tough monsters. The layout makes range combat and spells difficult. The monsters get stronger on the lower levels.
Somewhere in the range of 12 to 15 minimum for all classes.
Demons, Dread Beholders, Hill Giants, Beholders, Skulls, Wraiths, Ghosts, Trolls, Wyverns, Fire Para-Elementals, Fire Elementals, Acid Spheres, Liches, Demi Liches.