Nether Fortress

Nether Fortress.png.

Nether Fortresses are structures that are naturally generated in the Nether. They are bridge structures constructed of Nether Bricks, Nether Brick Fences, and Nether Brick Stairs. They are the only places where Nether Warts or Blaze spawners can be found. They are also the only place where either Blazes or Wither Skeletons can spawn.

Structure
Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can tunnel through Netherrack, and the inside of the tunnels will be cleared of Netherrack by the terrain generator. However, at broken bridge sections, the terrain is not cleared, which may create a tunnel that leads straight into a wall of Netherrack. Their main structure takes a shape of bridges with supporting pillars outside and corridors inside, both which have 3-blocks wide walking space. The bridges can be broken at the end. The interior of the structures have 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors. The rooms include:
 * Lava well rooms
 * Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
 * Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress
 * Rooms with four exits. Some of them could lead only to Netherrack.
 * Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

Spawning
Nether Fortresses may be spawned in inconvenient places, where half of or even the majority of the fortress is buried in Netherrack. The interior will not be filled, however; all hallways and passages will be clear except for open walkways and bridges. It is rare, but possible that Glowstone spawn inside of the fortress pathways.

Mobs
Nether fortresses spawn many of the rarer mobs of the Nether at a much higher rate, including Magma cubes. Nether fortresses also spawn many exclusive mobs which cannot be found anywhere else, which is one of the main reasons why people venture in the nether. These include: Blazes, Skeletons and Wither skeletons.

Blazes are desired because of their drop - blaze rods as well as the high amount of experience they give. Wither Skeletons are mostly desired for their Wither Skeleton Skulls, which help to create the Wither. Ultimately those who spawn the Wither are aiming to acquire the nether star which is used to construct the beacon.

The spawning algorithm checks if a chunk has a fortress within it. The algorithm then checks for the "bounding box" of the original fortress. It then allows mobs to spawn up to 7 blocks above the walkway, if there is a block beneath them. The entire width of Nether fortresses are taken into account, including junctions and special areas. Fortresses generated before 1.4 lost their bounding box when the fortress was unloaded for the first time due to a bug, and will not naturally spawn wither skeletons, skeletons, and blazes (apart from ones from spawners) after loading them again, even in newer versions that have fixed the bug

Any additions to fortresses will not spawn these rare mobs. Spawning is not affected by replacing the Nether brick with other materials.

Trivia

 * Often, Nether Fortresses spawn with no Nether Wart gardens in them at all.
 * The interiors of Nether Fortresses are safe from Ghasts, because they are enclosed and are made out of materials that Ghast fireballs can not destroy. However, the bridges are open to the sky, as are many platforms.
 * Nether Fortresses use a separate list of spawnable mobs from the rest of the Nether. This allows Blazes to spawn naturally and makes Magma Cubes much more common. Ghasts are not allowed to spawn, preventing them from being trapped inside.
 * Nether Fortresses will still be generated even if the "Generated Structures" option is turned off.
 * Mushrooms can spawn in Nether Fortresses.
 * Skeletons can spawn in Nether Fortresses. When the game tries to spawn a Wither Skeleton it has a 20% chance of spawning as a skeleton instead.
 * Glowstone can spawn in the corridors of Nether Fortresses.
 * Nether Fortresses which spawn high above the lava oceans of the Nether can create huge walls of Nether Brick twice as tall as Overworld mountains.
 * Nether Fortresses tend to spawn in "strips" aligning along the Z axis (north/south) in the nether, making them easier to find after one has already been found by simply moving north or south, keeping the X coordinate steady. This is a good method to use if you are having trouble finding Nether Fortresses (Tested over 200 seeds so far, the trend is strong in all tested seeds (Confirmed with AMIDST).)
 * Sometimes, two or more Nether Fortresses can spawn very close or even within each other, ultimately creating an even larger cumulative Nether Fortress.

Gallery
Nether-Festung Ruines du Nether Nether Fort Адские руины 下界要塞