The Fortress of Eternal Darkness is one of the two secret witch lots, which shipped with The Sims 2: Apartment Life. It is mainly for the witches with an evil alignment, as told in the lot's description and name. The Fortress of Eternal Darkness has a majority of dark stone walls and black floors, as well as a platform with the High Witch of Darkness's spell book, cauldron, and throne. There are also many spell books and cauldrons that can be examined or claimed.
If Seasons is installed, each season will be set to Autumn/Fall. It will rain most of the time.
Upon arrival, the High Witch of Darkness will usually appear by her throne, and then summon her Feline Familiar. You can then purchase reagents and potions and other witch needs, as well as schmoozing with the High Witch.
The Fortress of Eternal Darkness is usually a secret lot, but actually you can bring non-witch Sims in it. The game won't restrict non-witches into the lot even if it's a " secret lot".