Tower

Towers (called fortresses in New Super Mario Bros. Wii) are a type of level appearing in New Super Mario Bros. sub-series. 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 from New Super Mario Bros. as well as Rock-Candy Mines from New Super Mario Bros. U, which have two). The challenge of towers generally depends more on traps and obstacles than enemies.

Starting from New Super Mario Bros. Wii, some worlds have no towers whatsoever. World 9 of New Super Mario Bros. Wii, the Special Worlds of New Super Mario Bros. 2, Peach's Castle and Superstar Road of New Super Mario Bros. U are examples.

In Super Mario Run, the levels Cutting-Edge Spire and Scaredy Rat Race take place in towers.

New Super Mario Bros. boss fights
At the top of every tower, there is a boss fight with Bowser Jr., who becomes increasingly more difficult as Mario (or Luigi) reaches each tower. When he is defeated, he gets back up and jumps off-screen (and out of the tower). Additionally, the first time Bowser 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.
 * : The fight is simple, with no obstacles. Bowser Jr. runs at the player to attack.
 * : The fight is a bit harder, as Bowser Jr. now has the ability to jump high, and there is quicksand on the arena.
 * : Bowser Jr. can still jump high, and the fight is now held on a platform above a pool of water.
 * : Bowser Jr. changes his tactic; this time, he throws Koopa Shells, which need to be jumped on and tossed back.
 * : 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.
 * : The fight is on a see-saw, above a floor of spikes.
 * 2: The fight is on a see-saw, this time above lava, with Bowser Jr. throwing shells.
 * : Bowser Jr. throws shells, and the fight is on moving platforms.
 * : The fight is like a small obstacle course, with burners and moving platforms. Bowser Jr. throws shells.
 * 2: The final tower's fight is on yet another pillar above lava (complete with Lava Bubbles this time), still with shell-throwing added, and can jump high.

New Super Mario Bros. Wii boss fights
At the end of the fortresses, there is a boss fight with the Koopaling of that world. After defeating it, it flees to the castle at the end of its respective world for a rematch. In the last tower, instead of a Koopaling, Magikoopa is fought instead. After defeating him, he flees to the Airship. However, he is not fought there again.
 * : The fight with Larry Koopa is extremely simple, with small bottomless pits on each side of the room.
 * : Roy Koopa has a fight which is over a pit of quicksand.
 * : The fight with Lemmy Koopa is on a floor of ice with two bottomless pits on each side, similar to Bowser Jr.'s fight in in New Super Mario Bros..
 * : Wendy O. Koopa has a fight in a small room, similar to both her original fight in Super Mario Bros. 3 and Bowser Jr.'s in New Super Mario Bros..
 * : Iggy Koopa's battle has three shifting platforms.
 * : Morton Koopa Jr. has his fight over a pit of lava, with Skewers crashing down onto the side platforms.
 * : Ludwig von Koopa's battle has bottomless pits on each side like Larry Koopa.
 * : Magikoopa is fought on platforms coming in from the right of the room.

New Super Mario Bros. 2 boss fights
At the end of every tower, there is a boss fight with a group of Reznors. After defeating them, the Koopaling of the world swings in and taunts at Mario, except for World 6, when the game just cuts to the world map.
 * : Two Reznors on one wheel with four rectangular Coin Blocks.
 * '/': Four Reznors on one wheel with four rectangular Coin Blocks.
 * : Four Reznors on two wheels; two Reznors per wheel, with each wheel having four rectangular Coin Blocks. One bridge is positioned to the left of one wheel, while another bridge is positioned to the right of the other wheel.
 * : Four Reznors on two wheels; two Reznors per wheel, with each wheel having four rectangular Coin Blocks. One bridge is positioned between the two wheels.
 * : Four Reznors on one giant wheel with eight rectangular Coin Blocks.

New Super Mario Bros. U / New Super Mario Bros U Deluxe boss fights
At the end of the first six worlds' towers (excluding Rock-Candy Mines' second tower), there is a boss fight with Boom Boom, who, after Acorn Plains, has Magikoopa enhance his abilities. After Grinding-Stone Tower, the bosses of the towers Screwtop Tower and Slide Lift Tower are replaced by Boss Sumo Bro and Magikoopa respectively.
 * :Boom Boom in his most standard boss fight.
 * : Boom Boom given the power to spin jump. He keeps this ability in the following boss fights.
 * /: Boom Boom given the power to jump, similarly to Super Mario Bros. 3.
 * : Boom Boom enlarged.
 * : Boom Boom given the power to fly, similarly to Super Mario Bros. 3.
 *  2: Boss Sumo Bro, a Sumo Bro enlarged by Magikoopa, giving him the power to shock the entire ground.
 * : Magikoopa using magic blocks to crush the player.

Challenge Mode Original Courses
Tower-related Challenge courses that have no origin.
 * Fire, Bro! - Special
 * Three Dry Bones - 1-Up Rally

Super Smash Bros. series
A tower, particularly the Slide Lift Tower level, appears as one of the variations of the Mushroom Kingdom U stage in Super Smash Bros. for Wii U and Super Smash Bros. Ultimate, featuring a Slide Lift derived from said level. Like the other sections of Mushroom Kingdom U, different obstacles such as icicles and Water Geysers appear as stage hazards.

New Super Mario Bros. 2

 * European Website bio: "These annoying edifices block your path halfway through each World. You'll need to climb all the way to the tower's top before you can face the particular challenge it holds – on and up!"

Trivia

 * The seven castles on the overworld map in Super Mario World appear as towers.
 * In New Super Mario Bros. U, all the towers have Bowser Jr.'s face marked on the walls of the towers.
 * Due to both the tower icon and map model resembling fortresses in New Super Mario Bros. Wii, New Super Mario Bros. U, and New Super Luigi U, there is some slight naming confusion across languages:
 * The New Super Mario Bros Wii instruction manual refers to towers as "fortresses." However, the Prima Games guide refers to them correctly as "Towers".
 * In the Japanese supplemental materials of New Super Mario Bros. and New Super Mario Bros. Wii, towers were identified as forts. In New Super Mario Bros. 2, the name was revised to tower, matching the English versions. Despite this, the names for individual tower levels in the Japanese versions of New Super Mario Bros. U and New Super Luigi U continue to call them forts.