Fortress of Altitude

The Fortress of Altitude is Dr. Robotnik's most powerful creation in Sonic Underground. It is a giant flying fortress in the shape of an egg with eyes, making it very similar in appearance to the Death Egg.

Description
It is armed with many cannons, which can easily level an entire forest or village. The fortress is created with solid Mobibendum, a very strong substance that can withstand all of Sonic, Manic and Sonia's attacks with no damage at all. The metal weighs a ton, so making it fly would be impossible in normal circumstances, but Robotnik has managed to procure a rare Chaos Emerald to levitate it with tremendous ease, and the emerald also allows it to utilize the chaos energy to disintegrate any intruders who managed to infiltrate the fortress' hallways.

History
When Robotnik unleashed the fortress against the royal siblings, their attempts to retaliate were futile, and the forest they hid in was completely destroyed. They concluded that if the fortress is left as it is, it would completely wipe out the Resistance, leaving them no chance against the flying weapon of destruction. Robotnik then attempted to destroy Port Mobius for standing against him. When Sleet and Dingo interrupted him with over-flattering, he threatened to throw them off the fortress for interrupting his work, which led them to fear he would roboticize them for being no longer useful. They decided to rebel by stealing the Chaos Emerald from the fortress' engine room, thus causing the Fortress of Altitude to collapse into the ocean and explode.

Trivia

 * The name of the fortress serves as a reference to the Fortress of Solitude, the occasional headquarters of DC Comics' Superman.