Nether fortress

Nether fortresses are structures that are naturally generated in the Nether. They are bridge structures constructed of nether bricks, nether brick fences, and nether brick stairs. They are the only places where nether wart, blaze spawners and wither skeletons can be found.

Structure

Nether fortresses are very large complexes made mainly of nether bricks. Nether fortresses can tunnel through netherrack, and the inside of the tunnels will be cleared of netherrack by the terrain generator. However, at broken bridge sections, the terrain is not cleared, which may create a tunnel that leads straight into a wall of netherrack. Their main structure takes a shape of bridges with supporting pillars outside and corridors inside, both which have 3-blocks wide walking space. The bridges can be broken at the end. 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 room and corridors. The rooms include:

Consists of:

• 
  Nether Brick
• 
  Nether Brick Fence
• 
  Nether Brick Stairs
• 
  Soul Sand
• 
  Nether Wart
• 
  Chest
• 
  Blaze Spawner
• 
  Lava

• Lava well rooms
• Rooms with nether brick stairs in the center and soul sand at sides of the stairway containing two 2×5 rows of nether wart growing on it.
• Rooms with blaze spawners (normally 2 per fortress), which can have nether brick stairs leading up to them. The rooms also have nether brick fences.
• Rooms with four exits. Some of them lead only to netherrack.
• Rooms with a "stairway" of nether brick placed against a wall, leading up to the floor above.

In a nether fortress, there are two sections: an indoor section and an outdoor section. The indoor section has a nether brick roof and is where chests are spawned. The outdoor section has no netherbrick roof, although it can tunnel through netherrack, making it seem like it has a roof. The outdoor section is where blaze spawning rooms are found. In every fortress, there is one lava well room, which is the single connection between the indoor and outdoor sections.

Loot

Nether fortresses generate nether fortress loot with chests in the indoor sections placed at some interior points where corridors turn at right angles. Each chest contains 2-5 stacks of items, chosen from the following list. Chance listed is per stack, not for the chest as a whole.

Item	Quantity per stack	Chance
Diamonds	1 - 3	6.85% (5/73)
Iron Ingots	1 - 5	6.85% (5/73)
Gold Ingots	1 - 3	20.5% (15/73)
Golden Sword	1	6.85% (5/73)
Golden Chestplate	1	6.85% (5/73)
Flint and Steel	1	6.85% (5/73)
Nether Wart	3 - 7	6.85% (5/73)
Saddle	1	13.7% (10/73)
Golden Horse Armor	1	11.0% (8/73)
Iron Horse Armor	1	6.85% (5/73)
Diamond Horse Armor	1	4.11% (3/73)
Obsidian	2 - 4	2.74% (2/73)

Spawning

Nether fortresses may be spawned 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 spawn inside of the fortress pathways.

A blaze spawner generated into the side of some netherrack

Nether fortress tend to cluster together in strips that run north and south. When you're searching for your first nether fortress, traveling east or west will give you the best chances of running into one. If you travel north or south, you could travel a long distance between two strips of fortresses. But if you have already found a fortress, you should travel either north or south to find another, which will likely be within a few hundred blocks. Note that north and south spawning is just a tendency and you may not find additional fortresses simply by going north and south from a located fortress in every case. Be flexible in your search and note that additional fortresses may still be east or west from a located fortress.

Mobs

Nether fortresses spawn magma cubes at a much higher rate. They also spawn two exclusive mobs which cannot be found anywhere else, blazes and wither skeletons. They also have a chance of spawning normal skeletons, which are exactly the same as the Skeletons found in the overworld.

Blazes are desired because of their drop - blaze rods, as well as the high amount of experience they give. Wither skeletons are mostly desired for their wither skeleton skulls, which are necessary to create the wither. Ultimately those who spawn the Wither are aiming to acquire the nether starwhich is used to construct the beacon.

The spawning algorithm has two checks:

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, and that the block under the spawn location is nether brick.

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.

Video

https://youtu.be/BQJAebejadQ

• this post from http://minecraft.gamepedia.com/