Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: The bridges look just like that: large bridges. They have a 5 block wide pathway with a one-block-high wall on both sides, all composed of nether bricks. Instead of the solid nether bricks that corridors have beneath , they have large solid arches. Note that the one-block-high 'railing' does not prevent you from falling off! The bridges often end broken-looking or abruptly end. The tunnels, when they generate, cut through netherrack which means if the fortress generates partly encased in netherrack, the bridges will generate as tunnels with a nether brick floor and netherrack walls and ceiling. Be careful when wandering around here, because if you're not careful you could be thrown off the bridge by an explosion, knocked back by an attack, or attacked from the other side of the fortress by the marauding blazes and ghasts.