Panopticon
Panopticon, the Darksteel Citadel, is a fortress inside of the hollow plane of Mirrodin. Made from an indestructible metal called darksteel, from here the mad golem Memnarch controlled his artifact minions and watched over his world through the eyes of his creations, the myr. In his device, the Darksteel Eye, the world is viewable from the perspective of these artifact creatures.
Memnarch also had his Ascension Web built here by Slobad while he hibernated in anticipation of Glissa Sunseeker's return to the core of Mirrodin.
Trivia
In the real world the Panopticon is a special prison design.