Stairs are one way in which your team may move between levels. The other ways may include: ladders, poles, pits, and teleporters.
Stairs must be placed on a floor tile. Current stairs suit the wallsets of stone and the default. Stairs down must match a set of stairs up on the level below, that is they must have the same coordinates on the lower level.
Ladders or poles
Ladders or poles work the same way but are a wall object placed on walls tiles.
Currently poles suit, cave, stone, and the thicket wallsets. Ladders currently suit the stone wallset.