Wily Egg

The Wily Egg is a flying fortress jointly planned and constructed by Dr. Wily and Dr. Eggman and resides within the Skull Egg Zone. The overall shape is that of the Death Egg that Dr. Eggman has built before, but it possesses the imposing skull motif that all of Dr. Wily's fortresses are known by.

The interior is familiar fortress standards by both doctors, containing, as noted by Sonic and Mega Man, "Bridge beams and exposed superconductors" and "narrow platforms over armor-piercing spikes" along with slews of iconic mooks from both doctors' robotic armies.

Aside from the standard fortress defenses and the robot mooks, there are four major lines of defense leading up to the Egg-Wily Machine X and the final confrontation with the two mad scientists.
 * Line 1 is exterior and consists of an army of eighty one time-cloned Robot Masters plus Rouge Woman.
 * Line 2 is the first interior defense and consists of the Mega Man Killers.
 * Line 3 is the fearsome Chaos Devil.
 * Line 4 is the final line of defense and is composed of Bass, Treble, and Metal Sonic.
 * The final arena is where the Genesis Wave Unit is, where Sonic and Mega Man must battle the Egg-Wily Machine X.

After Worlds Collide
After the Super Genesis Wave was undone, the Wily Egg ended up in the Sol Zone and in the position of Captain Metal. He remodeled it into the Egg O' War.

Trivia

 * Originally Dr. Eggman wanted to call the fortress the Death Egg III, while Dr. Wily desired to name it the Wily Flying Fortress. After tying six rounds of Rock-Paper-Scissors, the two scientists agreed on the name Wily Egg.