World 8-Bowser Castle (NSMB)

World 8-Bowser Castle is the final level New Super Mario Bros., being Bowser's Castle. The Mario Bros, once inside, will encounter a massive amount of traps, including Rocket Engines and Thwomps. The level is the longest in the game, and some parts of it require pressing one of the many ? Switches to be found. These switches will flip the gravity of the castle upside down, and any enemy along with it. This action is required for getting to certain doors.

The second part of the castle is a long looping room, similar to the looping castles of 4-4 and 7-4 from Super Mario Bros. After that is the boss door.

Final Boss
Once inside the room, the player will encounter Bowser Jr. on a long bridge over a deep pit. He will toss his father's bones into the couldron, thus reviving him, much to Peach's horror.

The duo will then fight Mario together, with Bowser Jr. using his usual shell-tossing tactic, and his father breathing flames behind him. After the player defeats Bowser Jr, his father will increase his attacks greatly. Mario must then run under one of Bowser's giant jumps and hit the Skull Switch at the end of the bridge, ending the fight and winning the game.

Boss Trivia
There are multiple ways to win the final boss battle. If Mario or Luigi have either a Fire Flower, Blue Shell or Mega Mushroom, then Bowser Sr. can be defeated first, leaving Bowser Jr. to be easily disposed of via the Skull Switch. It is also possible to defeat both of them at once by running under one of Bowser's small jumps; however, this is extremely tricky because to do so requires being small and having excellent timing.