Nether Fortresses are generally made up of two levels: bridges and corridors. Bridges have pathways that are five blocks wide and framed with a wall that's one block high. If a fortress is embedded within Netherrack, the bridges spawn as tunnels instead. Corridors, on the other hand, spawn with 3 x 3 walkways that are surrounded by nether brick. They also have 2 x 1 nether brick fences that serve as "windows" for the structure. These fortresses are easily overlooked if your elevation is too high or you're at lava level below them. Fortunately, in either case, you can look for other fortress indicators like wither skeletons and nether bricks that indicate a nearby structure.