Most important thing for a duel map:
- small
- flat
- as few buildings as possible ;)
How do you figure that? Imagine: you spawn in a modest town square surrounded by buildings - all inaccessible but for two just ahead, side-by-side, with large torch-lit doors and hanging signs. They are a tavern and an Arena. Enter the arena and there are stairs up to the stands and stairs down to the pit. Enter the tavern and there is, well, tavern stuff.
People could just duel in the square and say to hell with the rest. Or, if they were interested, they could explore the other, more interesting spaces.
EDIT: Ruins is already large enough that, when there are only two people on, they end up typing "where are you"? anyway. And the answer is usually given with landmarks. "Near the little village." "In the center." etc. The new map would only be more useful in that way.
