Nether Fortress

A nether fortress, or simply fortress, is a large complex of bridges, corridors, and towers found in the Nether.

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 4 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, crimson trees, and warped trees to generate inside the fortress pathways.

Structure


Nether fortresses are large complexes, composed of nether bricks materials, that are supported by massive pillars that tower high above the lava oceans. 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 "broken" 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 5 plain four-way crossings.
 * Up to 4 four-way crossings with arches made of nether brick and nether brick fence.
 * Up to 4 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 spawner platforms: structures consisting of three full-block "stairs" leading to a small platform fenced with nether brick fence, with a blaze 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 5 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 3 stairways (made from actual stair blocks) leading downward.
 * Up to 2 three-way intersections with a small exterior balcony.
 * Up to 2 stairways leading up to an open landing, which have patches 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.

The actual area, which works as the fortress itself is the bounding box of the actual generated structure. The "bounding boxes" are based on the uppermost level of nether brick generated in the actual section (bridges, corridors, blaze spawner platforms etc.) except walls and railings on the side. Basically the level where you can walk, the level of the floor. The roof of sections with fully closed top behave like this as well. These are the base levels of the "bounding boxes". The "structure bounding box" is y +3/-2 calculated from each of these base level blocks. The outer, "(nether fortress) area bounding box" is y +3/-2 and x +-6 calculated from, again, these base level blocks. By entering the "area bounding box" at the first time, gives the player the "A terrible fortress" Achievement/Advancement. No fortress-exclusive mobs can spawn outside of the "area bounding box", and the "structure bounding box" is fully inside of this, just a part of it.

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.

In Bedrock Edition, instead of spawning anywhere within a structural bounding box, mobs spawn only on the northwest corner of various blocks scattered few and far between throughout the fortress. In order to identify these spawning spots, glass panes must be placed all over the fortress at the second block up from the ground.

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, 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.
 * Under the lava well, there is an opening out of the fortress, with a single block of nether bricks from which the lava spreads out.
 * For 1.16 Nether biomes, 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, crimson forest and warped forest huge fungi, nether wart blocks and warped wart blocks and crimson stems and warped stems can replace nether brick, most visibly along walls.
 * Nether fortresses, together with swamp huts, ocean monuments and pillager outposts, are the only four generated structures in the game that have special spawning rules within their bounding boxes.
 * Dungeons are not included, since the spawning condition is dictated by the spawner rather than the dungeon structure.

Gallery
Netherfestung Fortaleza del Nether Forteresse du Nether ネザー要塞 네더 요새 Netherfort Forteca Netheru Fortaleza do Nether Адская крепость Пекельна фортеця 下界要塞