When going up side stairs, the player will move up the screen (and down when they descend the stairs) in addition to moving across. For every tile of stairs the player passes sideways, they should also move up/down on the screen by 1 tile (i.e. they move diagonally on stairs). They will need to remain facing left/right, though. To ensure that they move correctly, you will need to create events that move the player in the right direction.