It's supposed to be that the further right on the map you go in Fortress mode, the 'deeper' you mine into the mountain. Realistically, floods would extend all the way to the right of the map and collide with the lava river in a great ball of steam if this were true in fortress mode, so fortress mode doesn't utilize the Z coordinate (up/down) like Adventure mode does; which is why adventure mode has up/down staircases.Adventure mode uses the Z coordinate, while Fortress mode does not.