Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: Corridors are the "indoors" part of a fortress. They are easily distinguishable by their 3x3 walkway encased in nether brick with 2x1 nether brick fences for 'windows'. Under the corridors solid nether brick generates to the surface. This is what makes the main 'fort' buildings of the fortresses: a set of nether brick corridors positioned above or around the surrounding bridges, with the solid nether brick to the surface giving it a fort-like look. Chests generate in the turns of these corridors and they can generate the all-important nether wart farm and blaze spawners. They are in most ways the safer parts of a nether fortress, because unless you've mined through the walls or ceiling you cannot fall off or be attacked from afar.