Fortress

Fortresses, also known as forts, are fortified buildings that appear in many Mario games, starting with Super Mario Bros. Usually, fortresses are found at the end of levels, behind Goal Poles. After reaching the fortress, Mario or Luigi can advance to the next level.

Super Mario Bros.
In Super Mario Bros., castles appear to be a fortress stacked upon a larger section.

Super Mario Bros. 3
In Super Mario Bros. 3, the fortresses that advance Mario to the next level do not appear. Instead, fortresses, also known as Mini-Fortresses, are usually found in the middle of worlds. They are levels with Boom Boom as their only boss, and he must be defeated to open Locked Doors and gain access to other parts of the world.

Super Mario World
In Super Mario World, fortresses are guarded by Reznors but are far less common, as there are only four in the game, and only one of them is mid-world. Fortresses are normally inaccessible after being completed, unless the player presses while holding  and  when at it on the map (does not apply to the Japanese version). The GBA version lets players re-enter fortresses freely after defeating Bowser; Dragon Coins have also been added to them. Like with castles, Yoshi cannot enter fortresses.

New Super Mario Bros. series
The end-of-level fortresses later return in New Super Mario Bros., New Super Mario Bros. Wii, New Super Mario Bros. 2, and New Super Mario Bros. U and reprise their earlier role from the first Super Mario Bros. game, allowing Mario, Luigi, and/or the Toads to continue to the next level. Black fortresses appear as parts of secret exits, except in the original New Super Mario Bros. Their mid-world role in Super Mario Bros. 3 is replaced by towers, which in New Super Mario Bros. Wii are called fortresses; however, one of the Mario Vs. Luigi courses in the original New Super Mario Bros. is a Fortress.

Yoshi franchise
Forts also appear in the Yoshi franchise as the mini-boss levels of each world.

Paper Mario
In Paper Mario, the Koopa Bros. Fortress appears in the game.

Super Paper Mario
Super Paper Mario includes Fort Francis.

Paper Mario: Sticker Star
In Paper Mario: Sticker Star, Goomba Fortress appears.

Other appearances
In Super Mario Galaxy 2, the Bowser Jr. levels at the end of Worlds 1, 3, and 5 are represented in the map as fortresses. However, only the first world's last level includes a fortress within it.

The Mushroom Kingdom stage in Super Smash Bros. Melee features a classic Super Mario Bros.-style fortress in the background. In Super Smash Bros. Brawl, Super Smash Bros. for Nintendo 3DS, and Super Smash Bros. Ultimate, one also appears in the Mushroomy Kingdom stage, in the same place as in World 1-1 of Super Mario Bros.; however, it is in the background, so fighters cannot enter it. It also appears in the Final Destination versions of the stage, as well as Ultimate's Battlefield version. Like the rest of the stage, it has a more realistic and withered appearance.

Fortresses return in Super Mario 3D Land but only in World 2-3 and Special 1-3, where they look as they did in Super Mario Bros. As in that game, they appear behind the levels' Goal Poles and can be entered, this time for a 1-Up Mushroom.

In Super Mario Run, Bowser's Fortress is a fortress replica that appears as a Kingdom Builder decoration. In World Tour and Remix 10, castles have fortress icons.

Appearances in other media
In Mario Ice Capades, during the whole show, King Koopa rode a moving fortress. A fortress also appeared in the background of the show as a prop. In The Super Mario Bros. Super Show!, a fortress appears only in the intro.

In Animal Crossing: Pocket Camp, the fortress as it appears in Super Mario Bros. could be obtained as special furniture in Fishing Tourney #12 (themed after the Mario franchise in celebration of Mario Day 2019), after passing the 4,900; 5,900; 7,100; 8,300; or 9,800 cm milestone.