Castle

Castles are the most commons hideouts of bosses in the Mushroom World. Small castles can be denominated Fortresses and castles cannot be confused with fortresses. Castles can be characterized being the home of some big bosses. Although in most games Castles look pretty much the same, they tend to change from game-to-game.

Super Mario Bros.
In Super Mario Bros., Castles are made of red bricks, and all of them have the same basic shape. The castles can be found at the end of each world, and inside the Player will encounter Bowser, which turns out to be a fake, as there's another Bowser waiting for him in the next castle. This will go on until Mario reaches World 8's castle, where he will find the real Bowser, and Princess Peach.

As Super Mario Bros.: The Lost Levels is almost identical to Super Mario Bros., Castles play the exact same role in this game, and look almost exactly the same too.

Super Mario Bros. 2
In this game, there is only one real Castle. It is located at the end of the game, and is inhabited by Wart, the game's final boss. The castle is unique among most castles in the Mario Series, looking like some kind of grand palace, instead of a battered fortress for the Boss.

Super Mario Bros. 3
Castles in Super Mario Bros. 3 are found at the end of every World, but will not actually be inhabited by a boss battle (except for World 8 which has Bowser as the boss), instead there will be the King of the World Mario is in, who has been transformed by one of the Koopalings who stole the Kingdom's Magic wand. After getting to the castles in this game, Mario will board one of the Koopalings' Airships, where they will find the boss of the world.

Super Mario World
In this game, like in Super Mario Bros., castles will be found at the end of each world. However, more like in Super Mario Bros. 3, one of the seven Koopalings will be guarding the Castle.

Super Mario 64
In Super Mario 64 and Super Mario 64 DS, the main hub world is Peach's Castle.

New Super Mario Bros.
In New Super Mario Bros, Castles work the same way they do in the original. The player does not fight False Bowsers, but faces other bosses, like Petey Piranha. They are filled with traps, and each are unique to each other. The last castle in the game is Bowser's Castle.

Mario Kart series
All the games of Mario Kart Series have had castle racetracks. Most of the Castle racetracks are based on Bowser's Castle, but some are a few are around Peach's Castle instead.

Super Mario World 2: Yoshi's Island
Like in Super Mario World, and most of the other games, the castles in this game are found at the end of the World the Player is in. However, the creature found at the end usually does not intend to harm Baby Mario and Yoshi, until Kamek comes, and transforms it into a huge boss for the Player to fight, trying to slow down or even stop Yoshi from reaching Baby Bowser's Castle. Once the player gets to Baby Bowser's castle, even though it is guarded by Kamek, the player does not get to fight Kamek; instead, the player will fight Baby Bowser, powered up with Kamek's magic.

Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG, near the beginning of the game, Mario will go to Bowser's Castle, here named Bowser's Keep, to rescue Princess peach from Bowser. Also in this game, castle-like areas will continue to appear in some places.

Paper Mario series
A castle of some form appears in the end of almost all the Chapters of Paper Mario, and its sequels, which rarely will have something else as the Boss' hideout.

Trivia

 * In the 2010 version of Nintendo Monopoly, there is a Brick Block card that rewards the player with $100 for inheriting a castle.