Have you tried killing one of the enemies? The purple door only opens from the other side. I guess I should have made the tutorial more clear.
Thanks! I think what threw me off is that when I got to the signpost with x and c I hadn't killed any enemies yet (I just avoided them), so I thought the signpost was telling me how to kill the enemies. A signpost for how to kill enemies would probably help anyone else who might have the same confusion.