The Egg Fortress is a gigantic egg-shaped fortress floating in the "weird bit" of the Special Zone that was the evil Doctor Robotnik's base of operations and a location that appears in the Sonic the Comic series published by Fleetway Editions. It was his main headquarters prior to and during his early years as planet Mobius' most notorious dictator.
Following Robotnik's second major defeat at the hands of Sonic the Hedgehog and the destruction of his first Death Egg satellite, Robotnik re-located his operations deep into the Special Zone where he could work undisturbed and hidden away from the heroic blue hedgehog. This new lair, the Egg Fortress, true to its name, is a large egg-shaped superstructure with smaller egg sub-structures connected to it, and it was where many of the Doctor's most devious creations were built, including the super-Trooper Predicto, Metallix and the Brotherhood of Metallix.
Robotnik attempted to destroy the Star Posts around Mobius, in order to ensure that nobody could reach his new base, although he was prevented by Sonic. Later, Sonic managed to salvage one Star Post for his secret underground base, which he used for quick transport around Mobius and into the Special Zone.
From his Egg Fortress, Robotnik took control of much of the Special Zone, including the sentient energy field known as the Omni-Viewer. He forced the Omni-Viewer to send Sonic and his friends into the future, and when they returned to Mobius 6 months later they found that Robotnik had managed to conquer the whole planet from his Special Zone base. Robotnik continued to manage his operations from the Special Zone, setting up various minions to govern the more important zones in his absence, such as the Marxio Brothers in the Casino Night Zone.
Robotnik also continued performing his experiments on his base, along with his chief scientist Grimer. When Troopers arrested Amy Rose for fraternizing with Sonic, she was brought to the Egg Fortress to be put inside a Super Badnik (although she was subsequently rescued by Sonic and Johnny Litefoot after they both were encased in a large bubble by the fortress' automatic defenses). Robotnik and Grimer also developed the Metallix robots in the Egg Fortress.
When Robotnik located the Floating Island, he briefly moved some of his operations to the Launch Base Zone on the Floating Island, in preparation for the launch of his new Death Egg, and he eventually abandoned the Egg Fortress altogether in favour of a more centralized base, the Citadel Robotnik, within the Metropolis Zone.
The fortress was still active, however, becoming base of operations for the Brotherhood of Metallix. From there, the Brotherhood hatched their plan to hijack the Omni-Viewer and create their own copy of it, thereby gaining the power to travel through time and space. Knuckles the Echidna and new friends, the Chaotix Crew, ventured here to retrieve Omni and put an end to the Metallixes, with Mighty the Armadillo raising the alarm by punching his way in. A lone Metallix was sent to tackle them, almost succeeding in defeating them until Knuckles disabled it for a while. The team were soon betrayed by Nack the Weasel, leading the heroes right into "the lion's den". When Nack himself was betrayed, a final conflict with the Metallixes led to Knuckles using Nack's disruptor gun to seemingly wipe them all out. After Chaotix abandoned the base, a back-up system activated which brought the robots back to "life". Without any interruption, the Egg Fortress was once more a secret base.
The Egg Fortress was left to run on automatic, creating an army of Metallix badniks with the purpose of creating the ultimate Sonic-form badnik. Eventually the Metallixes became self-aware, forming their own Brotherhood of Metallix, and decided to take over Mobius themselves. The Metallixes also brought their creator Grimer to the Egg Fortress and forced him to build the Alpha Device. Upon discovering that his own creations had failed him, Robotnik enlisted Sonic to venture to the Egg Fortress and rescue his scientist. After searching the acres of the base, Sonic eventually located the Emperor Metallix moments before they achieved their goals. Sonic stole the Alpha Device and defeated the remaining mooks, but relented when the Emperor threatened to crush Grimer. After this moment of weakness, the Brotherhood soon moved its operations to the Miracle Planet, and left the Egg Fortress abandoned and adrift in the vastness of the Special Zone. The fortress has not been used since and was presumably destroyed along with the entire Special Zone.