The Fortress was a building placed in the city of Arcopolis by one of the factions in an intergalactic war. It housed a weapon capable of not only destroying its target, but all evidence that the target had existed and anyone who knew about the target. This meant that anything destroyed by the weapon would effectively vanish. People destroyed by the weapon returned as ghost-like beings who pulled anything they touched out of reality. The weapon was cylindrical and was activated by touch. It was later revealed that the weapon did not function properly.
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)