Nether Fortresses spawn in all the biomes in the Nether. The trick is to find a grid/region that has one. Generally, each region in the Nether can only spawn one of two structures. You'll either find a fortress or a bastion remnant. If you've come across a bastion remnant in your wandering, you'll have to venture to another region to find a fortress. To help you visualize these regions, think of it this way: • Regions are made up of 432 x 432 blocks (Java) or 480 x 480 blocks (Bedrock). • Every region has a 4-block on the east and south borders to spawn structures. Keeping these figures in mind, you only have to cover a 368 x 368 (Java) or 416 x 416 (Bedrock) section of the region wherein possible structures may generate. Some players say that they can find fortresses if they follow the Nether's North/South axis. They're supposed to generate every 200 to 400 blocks. However, other players prefer to look for fortresses organically via exploration.