Nether Fortress

A nether fortress is a large structure found in the Nether, consisting of bridges, corridors and towers. Nether fortressess are the only place where wither skeletons and blazes spawn, the latter of which's blaze rods being required to craft and fuel brewing stands and craft eyes of ender to access the End. It also features nether wart as loot, which is a key ingredient in brewing.

Generation
Nether fortresses generate in all Nether biomes. To do so, the game splits the Nether into regions in which one of either a fortress or a bastion remnant can generate. The regions are 432×432 blocks $$ and 480×480 blocks $$.



Each region has a four chunk separation located on the south and east borders of the region in which neither a fortress nor a bastion can generate.



This leaves only a 368×368 block section $$ or 416×416 block section $$ where a structure can generate.

Two structures never generate in the same region, although they might overlap if they generate close to the separation border. $$, the chance of a fortress generating instead of a bastion is $$ (40%), while $$ the chance of a fortress generating instead of a bastion is $2/5$ (33.3%).

Nether fortresses can generate buried in netherrack. In such a case, the interior is not filled with netherrack; all hallways and passages are clear except for open walkways and bridges. It is possible but rare for glowstone or crimson and warped huge fungi to generate inside the fortress pathways.

Structure
Nether fortresses are large complexes composed of nether bricks that are supported by pillars that tower high above the lava seas.



The fortress generation starts with a plain four-way crossing centered at chunk coordinates 11, ~, 11 of the designated chunk.



A fortress has two areas, an exterior area of open bridges and an interior area of enclosed corridors. Both the bridges and corridors can end in a unfinished passway structure or may simply end without elaboration. Fortresses can tunnel through netherrack, giving the "exterior" areas an appearance of tunnels with nether brick floor and netherrack walls and ceilings. At broken sections the terrain is not cleared, which may create a tunnel that leads straight into a wall of netherrack.





The exterior consists of:
 * Straight bridges.
 * Up to five plain four-way crossings.
 * Up to four four-way crossings with arches made of nether brick and nether brick fences.
 * Up to four small rooms with a single entrance and full-block "stairs" leading to the roof, which may have a single path leading out.
 * Up to 2 blaze monster spawner platforms: structures consisting of three full-block "stairs" leading to a small platform fenced with nether brick fence, with a blaze monster spawner in the center.



The interior of the structures have 1×2 windows with nether brick fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors. Rooms include:
 * The lava well room, which is the connection between the interior and exterior areas.
 * Straight corridors.
 * Up to five four-way crossings.
 * Up to 20 corridor turns (10 right-turns and 10 left-turns), each with a $$ chance of having a loot chest in the corner.
 * Up to three stairways made from actual stair blocks, leading downward.
 * Up to two three-way intersections with a small exterior balcony.
 * Up to two stairways leading up to a garden of soul sand and nether wart at the base of the stairs, a corridor leading away from the upper landing and a corridor behind the stairs. If the room is generated embedded in netherrack, only one block above the landing is cleared.



Mobs
Fortresses use a list of possible mobs to spawn that is separate from the rest of the Nether, regardless of the biome the fortress generates in. This includes zombified piglins, skeletons and magma cubes, as well as two exclusive mobs not found anywhere else: blazes and wither skeletons.



In Java Edition, the spawning algorithm has two checks:


 * 1) It checks if the spawn coordinates are within the "bounding box" of a single piece (e.g. corridor or walkway) of the fortress. (referred as "structure bounding box" above) In this case the block type of the ground does not matter.
 * 2) It checks if the spawn coordinates are within the "bounding box" (referred as "area bounding box" above) of the entire fortress and whether the ground consists of nether bricks.

If either check passes, it uses the special mob list for fortresses rather than the list for the biome when choosing the mob to spawn. The actual mob spawning proceeds as normal for the mob chosen from this list.

The structure bounding box for the 4-way intersection is pictured above (top-down view), and consists of a 19 x 11 (height) x 19 volume centered on the floor block in the center of the intersection. This contrasts many of the other structure bounding boxes as their outlines tend to tightly follow the physical bounds of the structure.

In Bedrock Edition, instead of spawning anywhere within a structural bounding box, most mobs spawn only in structure spawn locations along varied-length lines spaced apart 4-11 blocks throughout the fortress. They are not set to a particular Y level other than "inside the structure" and are indeed columns several blocks high. They do not require any special type of block, any regular spawnable block will do. To identify these spawning columns, glass panes can be placed all over the fortress, 1 block above surface blocks. This will keep the mobs stationary. (This technique works in Bedrock due to mobs spawning in the northwest corner of blocks.) Note that these spots may be on top of the raised side blocks, so these side blocks have to be removed before a glass pane grid can be placed. Zombified Piglin spawns do not obey structure spawn locations; they can spawn anywhere in the fortress.

Mobs spawn at a much higher rate if the fortress is surrounded by soul sand valley or warped forest biomes, as hostile mobs in these biomes (ghasts, skeletons or endermen) spawn much less frequently, allowing most hostile mobs to spawn in the fortress.

Loot


Fortresses generate nether fortress loot with chests in the indoor sections placed at some corridor turns.

Trivia

 * In rare cases, a fortress may not generate any indoor rooms except one single lava room, forcing the player to go to another fortress (or housing unit bastion) to find their first nether wart. This usually occurs when a portion of the exterior section of the fortress blocks the exit to the interior section of the lava well room.
 * Sometimes, two or more fortresses can generate close to or even within one another, ultimately creating an even larger cumulative fortress.
 * There is an opening out of the fortress under the lava well, with a single block of nether bricks from which the lava spreads out.
 * The soul sand valley bone blocks and basalt pillars (down to within three blocks as-usual of a walkway) can replace more nether bricks. Less-often, huge fungi, nether wart blocks, warped wart blocks, crimson stems, and warped stems can replace nether brick, most visibly along walls.