One of the last features to add in my JSRL roguelike game is multiple levels. I implemented this feature today. You need stairs up and down. Of course the topmost level should not have a stairway leading up. And the lowest level should not have a stairway going down.
I wanted to make the stairway movement logical. That means if you go down a stairway to get to a lower level, then you should be right at the location in the lower level where the reverse stairway exists. It was a little tricky, but I got that right. You walk down the stairs to find yourself standing on the stairway that leads back up to the higher dungeon level.
The final major feature I need to add is monsters. I am saving that task for tomorrow.