Stage 9 brings a new graphics set (chemical plant) and our first multi-level design - pits with elevators linking the upper and lower sections, with some overlapping. It also heavily uses long corridors with fast-moving enemies that "see" you from any distance, making it very hard work to map their starting positions.
I went with some layering of the down arrows and Z-ordering as a depth cue, meaning sometimes there's an arrow in front of a map. Avoiding overlap means some of the down arrows are ridiculously long. I may have to review this scheme if later levels get more extreme.