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From the Minecraft wiki:

Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors.

The rooms include:

• Lava well rooms
• Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
• Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress Rooms with four exits. Some of them could lead only to Netherrack. Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

From the Minecraft wiki:

Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors.

The rooms include:

• Lava well rooms
• Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
• Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress Rooms with four exits. Some of them could lead only to Netherrack. Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

From the Minecraft wiki:

Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors.

 

The rooms include:

 

• Lava well rooms
• Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
• Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress Rooms with four exits. Some of them could lead only to Netherrack. Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

From the Minecraft wiki:

Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors.

The rooms include:

• Lava well rooms
• Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
• Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress Rooms with four exits. Some of them could lead only to Netherrack. Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

I found this from the Minecraft wiki in a matter of minutes:

Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors. The rooms include:

• Lava well rooms
• Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it. Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress Rooms with four exits. Some of them could lead only to Netherrack. Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

From the Minecraft wiki:

Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can often 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 flat 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 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors. 

The rooms include:

 

• Lava well rooms
• Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
• Rooms with Blaze spawners. Nether Brick Stairs can lead up to them. The rooms have Nether Brick Fences. There are normally 2 spawners per fortress Rooms with four exits. Some of them could lead only to Netherrack. Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.