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 space stations like the Wily Star as bases in the Game Boy games. In the first Mega Man game, Wily's base is called Dr. Wily's robot manufacturing plant (Ｄｒ．ワイリーのロボット大工場, roughly "Dr. Wily's giant robot factory"), but was renamed Wily Castle 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 battleships in the World series for the Game Boy. This is also the tallest building in the series, preceding the Jakob Orbital Elevator from Mega Man X8 and Neo Arcadia Tower from Mega Man Zero Series which possibly connection from there to the Later Series.
 * 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.