Adam Mitchell's fortress was the place Adam and the Tremas Master kept the Doctor's companions prisoners.

When the Eleventh Doctor found Neal Shaw on the Vault, Neal gave the Doctor his vortex manipulator, so that he could track Adam's location. Once the Doctor entered the fortress, Adam revealed that he was working with the Master, and revealed his plan: he would kill all of the Doctor's companions, except for one, who would be saved by the Doctor's choice. (COMIC: The Choice)

The Doctor, however, had contacted his past selves, who merged their TARDISes and materialised inside the fortress. When Frobisher revealed he was disguised in the form of Adam's remote detonator, the Master summoned his army of Autons. Frobisher then released all of the Doctor's companions, who helped them in defeating Adam and the Master. (COMIC: Endgame)