Nether Fortress

Nether fortresses are structures that are naturally generated 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) will have the best chances of finding one; those who travel north or south may travel for hundreds or thousands of blocks before discovering a fortress.

Nether fortresses may be generated 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 for glowstone to generate inside of the fortress pathways.

Mobs


Nether fortresses use a list of possible mobs to spawn that is separate from the rest of the Nether. Nether fortresses spawn magma cubes at a much higher rate, and also spawn two exclusive mobs which cannot be found anywhere else: blazes and wither skeletons. Normal skeletons have a 20% chance to spawn in place of a wither skeleton. Ghasts do not spawn in fortresses.

Blazes are desired because of their main loot—blaze rods, as well as the high amount of experience they drop. Wither skeletons are mostly desired for their wither skeleton skulls, which are necessary to create the wither.

The spawning algorithm has two checks: If either check passes, it uses the special mob list for nether 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 each piece (e.g. corridor or walkway) of any fortress.
 * 2) It checks if the spawn coordinates are within the "bounding box" of the entire fortress.

Structure


Nether fortresses are very large bridge-like complexes, composed mainly of nether bricks, that are supported by massive pillars that tower high above the lava oceans. A nether 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. Nether 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.

Loot


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

Trivia

 * Often, nether fortresses generate with no nether wart gardens in them at all. In some rare cases, a garden will have generated, but the nether wart will be partially or totally destroyed by falling lava.
 * In extremely rare cases, a nether fortress will not generate any indoor rooms except one single lava room.
 * Sometimes, two or more nether fortresses can generate very close or even within each other, ultimately creating an even larger cumulative nether fortress.

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