This location exists primarily or exclusively within the Sonic the Comic continuity.
Information in this article may not be canonical to the storyline of the games or any other Sonic continuity.
Robotnik's Retreat

Robotnik's Retreat as seen in the comics.

Robotnik's Retreat is a location that appears in the Sonic the Comic series published by Fleetway Editions. It is an old fortress in the mountains, far north of the Metropolis Zone. It is unknown how many times Robotnik has visited, but it has been openly stated that when the dictator ruled Mobius, one of his main reasons for going was to get away from Grimer.

Robotnik visited the retreat with Grimer when Chaos was on the loose and from there used his Chaos Resonator to attract the four lost Chaos Emeralds, the Master Emerald, and thus Chaos himself, who had already absorbed three of the Emeralds. Chaos soon arrived, with Knuckles and the Freedom Fighters close behind. From within Chaos however, Sonic returned from the ancient past, too late to stop the monster absorbing the Emeralds and assuming his Perfect form.

Chaos' first step was use his power to reshape the retreat into a form more to his liking, and soon the monster was battling with Sonic. Events came to a head when Ebony and the dying Super Sonic joined the fray. As Super absorbed all the energy from Chaos to save himself, the retreat was reduced to rubble by the excess chaos energy. It is unknown what has become of the retreat's remains since then.