Currently, the moment you touch the stairs you go the next level. Usually, that's exactly what you want. Sometimes however you might want to spend a bit more time on the level. A couple of times, usually due to combat situations, I have inadvertently stepped on a stair tile when I didn't want to leave the level just yet. It might be worthwhile to add a toggleable option to give a Y/N prompt before leaving a level. This is consistent with most other roguelikes, where stepping on an exit tile isn't enough; you usually have to hit a command to leave the level (even in games with no backtracking like DoomRL). Obviously, this is super low priority.
