Wily Castle

Wily Castle (ワイリー城), also known by other names such as Wily Fortress, Wily Stage, Skull Castle and Wily Tower, is the common name used to Dr. Wily's several fortresses with skull motif that appear as the final stages of most games from the original Mega Man series, though not all of them have names. They often have several hazards, mazes, strong enemies, a rematch against the Robot Masters and a final fight against Wily inside one of his machines, like the Wily Machines and Wily Capsules.

Other fortresses
Besides the Wily Castle, there are other fortresses in the series as well: Cossack Castle (コサック城) from Mega Man 4, Proto Castle (プルース城) from Mega Man 5, "Mr. X's castle" from Mega Man 6, and Dr. Wily's space stations from the Game Boy games, like the Wily Star. The base of Lord Wily, Dr. Wily's counterpart from the MegaMan Battle Network series, is called Castle Wily in MegaMan Battle Network 3.

Trivia

 * Mega Man 10 marks the first appearance of Wily's Castle being (at least part of) in space. Until this point, the only fortresses in space were the fortresses in the World series for the Gameboy.
 * The Wily space station of Mega Man IV bears some resemblance to the title starships of the classic Japanese animes Space Battleship Yamato and Arcadia of my Youth.