You would have to modify the doors Enter(atom/movable/O) proc, so as it checked what direction you are coming in from (Just use if(get_dir(src, O) == SOUTH)).
With Enter() you simply return 1 if you want them to enter, and return 0 if you don't.
Look up enter in the reference for more info.
With Enter() you simply return 1 if you want them to enter, and return 0 if you don't.
Look up enter in the reference for more info.