Bowser's Castle

From Nintendo

Bowser's Castle is where most of the final battles with Bowser takes place in the Mario Games. The first game that the castle appeared in was Super Mario Bros. for the NES, here, a fortress was at the end of every world, until the last which was Bowser's Castle. The second appearance was in Super Mario Bros. 3, where it was in the Dark World. After that, the next game was Super Mario World, where there was a big sign that said Bowser right outside of it. It also has a secret back door that allows you to fight Bowser much sooner instead of going through all of the sections of the castle.