In “Vertical Scale,” we consider stairs which incline at angles greater than 45 degrees. At the DM’s disgression, such steepness impacts movement and melee combat.
At vertical rises of 15 and 20 feet over ten horizontal feet, the distance traveled is 18 and 22 feet. For either, we round to 20 feet of movement.
Considering also the extra effort to step up and, in the 20-foot case, a vertiginous decent, we justify halving the explorer’s move rate. So, moving up or down stairs—a ten-foot square on the map—costs 40 feet of movement.
Moving faster, an explorer must roll his or her dexterity score or less on a twenty-sided dice or tumble to the bottom of the stairs, taking d6 damage for each ten feet fallen.
If your chosen rules do not address the issue, add 1 to attack rolls for melee combatants on higher ground.
When a melee combatant suffers a violent blow (i.e. takes damage), he or she must roll against dexterity or fall and suffer damage as above.