The Fortress was a black pyramid placed in the city of Arcopolis by the Time Lords during an intergalactic war. Guns bristled on the ramparts and turrets, and the brutal black spikes contrasted heavily with the sleek architecture of Arcopolis.
The Fortress was powered by a weapon inside of it, which enabled it to project holograms and fire death rays as automatic defences. The Time Lords were afraid that, if they gave the Fortress a brilliant artificial mind, it would use the weapon and rise up against them, so instead of the capacity to find the best solution to any problem, they gave it a list of every possible problem and the best solutions.
A group of Eyeless landed on the planet and tried to take the weapon. The Tenth Doctor defeated them and destroyed the weapon. (PROSE: The Eyeless)