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. The amount of stages vary depending of the game, having from one to six stages.

Other fortresses
Besides the Wily Castle, Dr. Wily also used a Wily Station (ワイリーステーション) in the first four Game Boy games (two of them being space stations), the Wily Battleship (ワイリー戦艦) in Mega Man IV, and the Wily Star in Mega Man V. In the first Mega Man game, Wily's base is called Dr. Wily's Robot Manufacturing Plant, but was renamed Castle Wily in Mega Man Powered Up. 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 stations and battleships in the World series for the Game Boy. This is also the tallest building in the series, preceding other buildings like the Jakob Orbital Elevator from Mega Man X8 and the Neo Arcadia Tower from Mega Man Zero.
 * The Wily Battleship of Mega Man IV bears some resemblance to the title starships of the classic Japanese animes Space Battleship Yamato and Arcadia of my Youth.