Tower

Towers are the New Super Mario Bros. and New Super Mario Bros. Wii equivalent of the Fortresses from Super Mario Bros. 3 and Super Mario World. There is one in every world of the game (except for Worlds 6 and 8 in New Super Mario Bros., which have two). Unlike Fortresses, they are vertically-scrolling levels. They are generally short on enemies but very, very heavy on traps.

New Super Mario Bros. Boss Fights
At the end of every tower, there is a boss fight with Bowser Jr. Every fight is different in some way, as follows:

World 1: The fight is extremely simple, with no obstacles and a weak Bowser Jr.

World 2: The fight is a bit harder, since Bowser Jr. can jump, and there is quicksand.

World 3: Bowser Jr. can still jump, and the fight is above water.

World 4: Bowser Jr. changes his tactic; this time, he throws Koopa Shells which need to be tossed back.

World 5: Bowser Jr. doesn't throw Shells, but the fight is on a pillar of ice over a bottomless pit.

World 6: In the first tower, the fight is on a see-saw above a floor of spikes. In the second tower, the fight is on another see-saw above lava, with Bowser Jr. throwing shells.

World 7: Bowser Jr. throws shells, and the fight is on moving platforms.

World 8: The first tower's fight is like a small obstacle course, with Rocket Engines and moving platforms, plus Jr. throws shells. The last tower's fight is on yet another pillar above lava (complete with Podoboos this time), still with shell-throwing added.

New Super Mario Bros. Wii Boss Fights
At the end of every tower, there is a boss fight with the Koopaling of that world. After defeating them, they flee to the castle at the end of their respective world for a rematch. The fights are as follows:

World 1: The fight is extremely simple, with small bottomless pits on each side of the room and a weak Larry Koopa.

World 2: Roy Koopa has a fight which is over a pit of quicksand.

World 3: The fight is on a floor of ice with two bottomless pits on each side, and Lemmy Koopa creates balls that attempt to knock the players down said pits.

World 4: The Koopaling for World 4 hasn't been confirmed yet but the fight might be over a pit of water.

Unknown World: Morton Koopa Jr. has his fight over a pit of lava, with Podoboos jumping in each side of the pit.