Bowser's Castles

Bowser's Castles, also known as final castles, are special levels found around the end of most Newer Super Mario Bros. games. They're often found as the final "story" levels of their respective games, where Bowser is fought and Princess Peach is saved, though extra "non-story" content can usually be unlocked after completing one.

There are two notable Bowser's Castles which do not follow this description, being Bowser's Castle from Newer Super Mario Bros. Wii and World 9-8 from Another Super Mario Bros. Wii.

Bowser's Castle from Newer Super Mario Bros. Wii is initially built up as the final story level of its game, though once the player reaches the castle's end, no boss is fought, leading to a simple Goal Pole unlocking Koopa Core, home of the real final story level, Bowser's Keep. (Factory Tower is a clue to the bait-and-switch here.)

World 9-8 from Another Super Mario Bros. Wii is a sort of expansion of World 8-, the true Bowser's Castle of the game, though is not part of the final story level itself. Instead, it's just a simple level unlocked in the post-game World 9 by collecting all 30 Star Coins in World 8. Bowser is not fought again at its end, with a thank you message written in coins and a Goal Pole appearing in its final room.