I wonder what the etiquette for oncoming traffic was, those spiral staircases with a central spine aren't really walkable anywhere but at the outer wall.

Whoever has to step to the spine side would probably want a hand on the spine, palm making contact from the uphill side no matter wether facing up or down. So for someone uphill, the spine contact would be made with the outside hand (arm crossing in front), for someone downhill with the inside hand. On a clockwise staircase, this would leave the right hand comfortably idle for candle, tool or whatever the person deferring to (presumably higher ranking?) oncoming traffic on the wall side is carrying.