[Radek]

IMHO, placing a portal near the entrance into the Skull Castle (inside the dungeon) is the most effective way of solving problems with the dungeon. You can teleport to your portal from anywhere in the dungeon.
If you are inside the dungeon already, then do as suggested above. Jump into the water pool, save the game and try the exits from there. (Warning! All exits are "one way".) Take the exit where you find "manbeasts" attacking you and a giant (Gos'hin). Take this way. Wrong exits (as far as leaving the castle is considered) are exits with vampires, a small maze and a spell casting machine.