Tower

Towers are the New Super Mario Bros equivilent 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, which have two). Unlike Fortresses, they are vertically-scrolling levels. They are generally short on enemies but very, very heavy on traps.

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 chandileer above a floor of spikes. In the second tower, the fight is on a chandileer above lava, with Bowser Jr. throwing shells.

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

World 8: The first tower's fight is like a small obstacle course, with torches and moving platforms, plus Jr. throws shells. The last tower's fight is on a small platform above lava, still with thrown shells added.