Bowser's Castle, also known as the Bowser Castle, is a sinister fortified palace that is the main base of operations for the evil king Bowser Koopa (better known as Bowser) who also calls this place home, and it even serves as the capital of the Koopa Kingdom. It is one of the well-known recurring locations in the Super Mario Bros. universe. It comes in its many castle version s depending on which Super Mario game. The castle has often been destroyed and rebuilt, and appears different in each game that it appears in. It is usually filled with lava pits, booby traps, and Bowser's minions, and always includes a room where Bowser holds residence and that is sometimes the royal throne room in some Super Mario incarnations complete with the royal Koopa Throne which is only for those of the royal Koopa family including King Bowser and the Koopa Kids (also known as Koopalings) to occupy and assume full leadership of the entire Koopa Kingdom. In several of the storylines, Princess Peach is held captive there. The castle is frequently the last level in many games in the series. It is also known as "Bowser's Keep" in Super Mario RPG: Legend of the Seven Stars, "King Bowser's Castle" (in both Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3), and it also referred to as "Castle Koopa", "Koopa Castle", and "Neon Castle" in the Super Mario cartoons. The castle itself is always guarded by Bowser's most powerful servants and it is usually the final area in the Mario games that Mario and his brother Luigi and their friends visit to take on King Bowser and rescue Princess Peach Toadstool of the Mushroom Kingdom.
In Super Mario Bros. 3, not only that Bowser's Castle, called "Castle of Koopa" (also "Castle of Kuppa" in the videogame's Japanese version) was the game's final destination once again, but an entire world was built around it, called the "Dark Land". This 8th and final level was a subterranean valley reminiscent of the classical image of hell: with lava, floating skulls, and intermittent darkness. Thanks to the addition of an Overworld map screen, Bowser's Castle also had a defined exterior structure: it was built similar to a medieval fort, with a likeness of Bowser sculpted onto the front entrance.
In Super Mario World, Bowser's Castle is located in the heart of the archipelago, directly underneath the sea. The entire area is named the Valley of Bowser for this reason. The castle itself is located to the north. It had changed appearance yet again, this time resembling Windsor Castle, but noteworthy for the massive sign positioned directly over the entrance which reads "Bowser" in bright neon lights. Bowser himself is seen hovering over the topmost spire in his flying Koopa Clown Car.
In Super Mario RPG: Legend of the Seven Stars, Bowser's Castle rendered in 3-D for the first time, it was large and imposing consisting of large towers and rooms and also giant spade-modeled sticks on the castle's bailey. The castle was stationed on a mountain carved in Bowser's image above the Vista Hill which is oddly extremely close to Mario's own house. The castle was filled with minions and lava pits along with dark twisting corridors. There were also rooms containing challenges such as navigating across an invisible bridge suspended over lava, and jumping over barrels thrown by one of the Chained Kongs (in homage to Donkey Kong).
In the Super Mario spin-offs, the Bowser Castle can also serve as a go-kart course, a tennis court, a baseball stadium, etc., depending on the sport the game is based on.
- In the end credits of Super Mario RPG: Legend of the Seven Stars, Bowser's Keep was used part of the victorious parade with both the Hammer Brothers and King Bowser joined with the others.
- There was an original design of Bowser's Keep as a mere medieval fort on a mountain without the likeness of Bowser's face at the surface of the mountain beneath the castle during the early production of Super Mario RPG before the final design was made.
- The Paper Mario version of Bowser's Castle has made a cameo appearance in the Mario Kart: Super Circuit course Rainbow Road.
- In both Nintendo Comics System and Nintendo Adventure Books, Bowser's Castle was infrequently seen, and was shown to have a labyrinthine structure, possessing various passages, bridges and doorways.
- In Mario Golf: Toadstool Tour, a course known as the "Bowser Badlands" is located near Bowser's Castle.
- According to an article on Movoto.com, a real-life version of Bowser's Castle based on its appearance in Super Mario Bros. 3 would be 212 stories high, 52 stories taller than the tallest building in the world, the Burj Khalifa. However, due to the assumption that the castle should be merely 60 inches wide, it would cost only $455,000.
- In Mario Kart DS, There is a bug where at the start, the position will say 7th instead of 8th.
- In New Super Mario Bros. U, after King Bowser and the Koopalings including Bowser Jr. were successfully took over the Mushroom Castle in their attempt in making it the new capital of the Koopa Kingdom while Princess Peach Toadstool herself is being held hostage inside and Bowser's Koopa Airship hovers the peaceful palace, the Magikoopa leader Kamek on the evil royal family's behalf later cast a spell to create a dark purple tornado surrounding the castle as Bowser began making "changes" on his new domain in his image before the Super Mario Bros. finally defeated and expelled both Bowser and his nasty children from the Mushroom Castle as it finally changes back to normal. There are changes described on the castle in the game:
- The castle itself remains unchanged from the outside (it has changed on the inside, though), but the ground is charred and is surrounded by a moat of lava.
- The interiors of the castle has been magically twisted with certain changes suited to the royal Koopa family's likings with some of the objects inside in a similar fashion resembling those of the infamous Bowser Castle, such as huge spiked pillars and pilasters with some pillars having claw-like chapiters, darkened archs with metallic truss-like balcony posts, walls with bricks sticking out, molten lava moats, many wall torches, and giant magenta/purple banners bearing the iconic symbol of the Koopa Kingdom. Also, somewhere in the castle lies a hidden, spiked green Koopa Door holding 2 gold ring pulls on a bricked riddled wall with 2 wall torches and a Bowser statue head resting above the door. The door itself leads to a dome-like royal sanctum chamber with Bowser's statue on a claw-like chapiter at the heart of the room where the heroes beat Bowser the first time on the bridge before leaving the chambers and heading up the stairs towards the castle's bailey for the final battle with Princess Peach held captive at one of the castle's nearby towers.
- The castle is sometimes referred as the "Bowser Palace", "The Kastle", "Bowser Keep", "Castle Bowser", and "Bowser' s Palace" by some Super Mario fans.
- His castle architecture in Super Mario Odyssey bears a resemblance to Doctor Wily's Castle in Megaman 6.