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 fortress. Upon finding a fortress, it is best to travel in the North/South directions to find more Nether Fortresses along the same stripe.
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.
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 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.
|Nether Brick Fence|
|Nether Brick Stairs|
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:
- 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.
- It checks if the spawn coordinates are within the "bounding box" 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 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.
Fortresses generate nether fortress loot with chests in the indoor sections placed at some corridor turns. In Java Edition, each fortress chest contains 2–4 item stacks, with the following distribution:
|Stack Size||Weight||# Items||Chance||# Chests|
|Golden Horse Armor||1||8⁄73||0.329||29.1%||3.4|
|Flint and Steel||1||5⁄73||0.205||19.0%||5.3|
|Iron Horse Armor||1||5⁄73||0.205||19.0%||5.3|
|Diamond Horse Armor||1||3⁄73||0.123||11.8%||8.5|
|Icon||Advancement||In-game description||Parent||Actual requirements (if different)||Namespaced ID|
|A Terrible Fortress||Break your way into a Nether Fortress||Nether||Enter a Nether fortress.|
|1.0.0||September 16, 2011||Jeb tweets a teaser screenshot of changes to the Nether terrain generation, including fortresses.|
|September 19, 2011||Jeb releases a second teaser screenshot, showcasing the structure of the upcoming fortresses.|
|September 21, 2011||Jeb mentions that he is finished with most of the new Nether features, including two new mobs, one of which is a "retexturing attempt".|
|Beta 1.9 Prerelease||Added fortresses.|
|1.4.2||12w36a||Wither skeletons and skeletons now naturally spawn in fortresses.|
|Other mobs also found in fortresses spawn less often due to that addition. Zombie pigmen became remarkably more uncommon though due to their weight being halved.|
|1.6.1||13w18a||Chests now generate.|
|1.8||14w27a||Fortresses are now affected by the "Generated structures" option.|
|14w30a||Added possibility of obsidian in chests.|
|1.9||15w43a||Reduced loot in chests from 2–5 stacks to 2–4.|
|Pocket Edition Alpha|
|0.12.1||build 1||Added fortresses.|
|0.15.0||build 1||Horse Armor can be found in chest loot.|
|Legacy Console Edition|
|TU7||CU1||1.0||Patch 1||Added fortresses.|
|TU9||Changed fortresses to make nether wart rooms more likely.|
|TU19||CU7||1.12||Chests now generate.|
|New Nintendo 3DS Edition|
- In extremely rare cases, a fortress might 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.
- The description of the advancement "A Terrible Fortress" is "Break your way into a Nether Fortress"