Tower

Towers are a type of level appearing in New Super Mario Bros.. They are more or less the equivalent of the Fortresses from Super Mario Bros. 3 and Super Mario World. Unlike most Fortresses, in which the player may have to progress from left to right or vertically, Towers exclusively feature vertical direction. One appears midway through every world (except for World 6 and World 8, which have two). The challenge of Towers generally depends more on traps and obstacles than enemies.

In New Super Mario Bros. Wii, all the Fortresses have tower-based properties, being all vertical than horizontal.

New Super Mario Bros. Boss Fights
At the end of every tower, there is a boss fight with Bowser Jr., who becomes increasingly more difficult as Mario (or Luigi) reaches each Tower. When Jr. is defeated, he gets back up and jumps off-screen (and out of the Tower). Additionally, the first time Jr. is defeated in each tower, a brief cutscene ensues on the map, showing him running with Princess Peach to the Castle of that world. Some worlds have 2 towers.

World 1: The fight is simple, with no obstacles. Bowser Jr. runs at the player to attack. World 2: The fight is a bit harder, as Bowser Jr. now has the ability to jump, and there is quicksand on the arena. World 3: Bowser Jr. can still jump, and the fight is now held on a platform above a pool of water. World 4: Bowser Jr. changes his tactic; this time, he throws Koopa Shells, which need to be jumped on and tossed back. World 5: Bowser Jr. doesn't throw shells, but the fight is on a pillar of ice, which reduces the player's traction, and is held over a bottomless pit. World 6 (1st Tower): The fight is on a see-saw, above a floor of spikes. World 6 (2nd Tower): The fight is on a see-saw, this time above lava, with Bowser Jr. throwing shells. World 7: Bowser Jr. throws shells, and the fight is on moving platforms. World 8 (1st Tower): The fight is like a small obstacle course, with Rocket Engines and moving platforms. Jr. throws shells. World 8 (2nd Tower): The final Tower's fight is on yet another pillar above lava (complete with Podoboos this time), still with shell-throwing added.