Eggman constructed the Egg Fort under his base. He used it to track down the Chaos Emeralds. Sonic and Tails followed it with the Tornado-1, but the plane was shot down. Eggman obtained the green Chaos Emerald from the S.S. Anne and the Egg Fort was subsequently chased through a storm by Sonic and Tails in their new Tornado 2. This time the heroes take it out.
Eggman later made it mobile and used it again, but was again proven useless. He abandoned it for the Egg Fort II.
Eggman replicated the Egg Fort under his original base, expecting the President to order an attack. When Rouge, Knuckles and Topaz did launch an attack, he boarded it and used it in episode 15 to attack Station Square. However, Sonic destroyed it.
He later made it mobile and used it again in three more episodes, but was again proven useless. In "Cruise Blues", Eggman abandoned it for the Egg Fort II. What was left was salvaged by G.U.N. and reconstructed into the Gun Fort, using it to pit against Eggman. However, it was defeated and either recovered or rebuilt into the Grand Egg Imperial.