Nether Fortress



Nether Fortresses are structures that are naturally generated in the Nether. They include bridge structures constructed of Nether Bricks, Nether Brick Fences, and Nether Brick Stairs. They often contain Blaze spawners, and are the only place where Nether Wart can be found.

Nether Fortresses are comparable to the overworld Strongholds, but use different substances and do not contain End Portals.

Structure
Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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
It is also possible for Nether Fortresses to 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. Although, it is rare but possible that Glowstone spawns inside of the pathways.

Mobs
Nether fortresses spawn the rarer mobs of the Nether at a much higher rate. They can spawn Blazes, Magma Cubes and Wither skeletons, along with Zombie Pigmen.

The spawning algorithm checks if a chunk has a fortress within it. it 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.

Any additions to fortresses will not spawn these rare mobs. Spawning is not affected by replacing the Nether brick with other materials. you can also place nether brick around the fortress and that will increase the wither and other mob spawn area.

Trivia

 * The interiors of Nether Fortresses are fairly safe from Ghasts, because they are enclosed and are made out of materials that Ghast fireballs can't 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.
 * Skeletons that spawn in Nether Fortresses will not attack the player.
 * Glowstone can spawn in Nether Fortress corridors.
 * Nether Fortresses which spawn high above the lava ocean 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.