Chashio, SteffenBrand and chick, thank you for the feedback!

I see what you mean about choosing either 'god' (nothing has perspective) or 'aeroplane' (everything has perspective) view and not mixing the two. I had a go at some 'aeroplane' style walls and, well they work I think. But it's very time consuming to do - I need nine different tiles for one wall set, to account for each possible perspective. Which means nine more for each wall set I do, plus nine versions of each archway, nine versions of each door... I will have to decide if it's worth it for my goal, which is to (relatively) quickly make acceptable-looking dungeons for my players.

Then there is the question of how to make things stand out when you can't use colour...!

Here is how the perspective view came out:
perspective walls.png