Stairs are a connection of walls with gradual height difference. A maximum length of 5 tiles can be drawn, however they can be shorter. Stairs can connect walls with the ground, however they can do so with walls with different height (e.g. low walls and normal walls).
Just like walls, stairs can be damaged with rocks. Segments can either lower in height or disappear, restricting access between their two endpoints. Stairs can only attach to walls, but not to gatehouses and towers. They can host braziers.
Considering normal elevations, a unit will only get height advantage, if it stands on the highest end of the stairs.