Latest:

New Petra-inspired background; some slight detail fixes in fg; and a try at more convincing water ('wavy' reflections).
Also back to full color: I've realized I had a number of time-costly palette mistakes in the greyscale versions that were just getting out of hand due to the growing number of layers.
Petra references
here; and
this one in particular for what I'd like the more natural parts of the island to look like (possibly too much detail there considering the scale I'm working at and the available space).
The idea of the rounded rocks is (was?) to provide a contrast between Boecklin's harbour and the rest of the island. This contrast was supposed to be built through gradients of light, detail and rock shapes: brighter, more angularly man-made and detailed in front. I now see plainly that the previous versions shapes were out of character, esp in the first version it looks like a different image altogether. Hopefully it's better integrated now.
I see what you mean now about the map vs projection but I'd reckon the idea will be lost on the great majority of viewers and I think the image of the skull only becoming apparent from the one viewpoint, Holbein style, might be more effective.
Not sure we understand each other there

My point is not
idea here but visual consistency: I see the perspective as wrong if the skull shape isn't flattened down to the iso plane.
Are you suggesting that I'm losing readability this way? (I don't think you do, but I'd rather be sure!)