You will gain access to this dungeon during the Creature Of Fenkenstrain quest. It is used to gain access to the Mausoleum during the quest. Afterwards, this dungeon may appear to be useless, however the monsters inside can be great for training. The 'Experiments' have about 100 hitpoints and very low attack, strength and defense. This makes for very fast experience.
Please note that the Horsewoman Experiments that hold keys cannot be attacked after the quest.