Nether Fortress

Nether fortresses, or simply fortresses, are bridge structures found in the Nether.

Generation


Nether fortresses tend to cluster together in strips that run along the Z-axis (north and south). Players who travel along the X-axis (east or west) have the best chances of finding one; those who travel north or south may travel for hundreds or thousands of blocks before discovering a nether fortress. Upon finding a nether fortress, it is best to travel in the North/South directions to find more nether fortresses along the same stripe.

Nether fortresses may be generated in inconvenient places, where half of or even the majority of the fortress is 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 rare but possible for glowstone to generate inside the fortress pathways. The biomes they generate are:
 * Nether wastes
 * Soul sand valleys
 * Basalt deltas

Structure


Nether fortresses are very large complexes, composed of nether bricks materials, that are supported by massive pillars that tower high above the lava oceans. 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 $1/3$ chance of having a loot chest in the corner.
 * Up to 3 stairways (made from actual stair blocks) leading downwards.
 * 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.

Mobs


Fortresses use a list of possible mobs to spawn that is separate from the rest of the Nether. Fortresses spawn magma cubes at a much higher rate, and also spawn two exclusive mobs not found anywhere else: blazes and wither skeletons. Normal skeletons and zombie pigmen can also appear. Endermen or ghasts do not spawn in fortresses.

The spawning algorithm has two checks: If either check passes, it uses the special mob list for fortresses rather than the general list for the Nether when choosing the mob to spawn. The actual mob spawning proceeds as normal for the mob chosen from this list.
 * 1) It checks if the spawn coordinates are within the "bounding box" of a single piece (e.g. corridor or walkway) of the fortress. In this case the block type of the ground does not matter.
 * 2) It checks if the spawn coordinates are within the "bounding box" of the entire fortress and whether the ground consists of nether bricks.

Loot


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

Trivia

 * In extremely rare cases, a fortress may not generate any indoor rooms except one single lava room.
 * Sometimes, two or more fortresses can generate very close or even within each other, 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.

Gallery
Netherfestung Fortaleza del Inframundo Forteresse du Nether ネザー要塞 네더 요새 Nether Fort Forteca Netheru Адская крепость 下界要塞