This looks great mthomas - well done. I like your doors. I like your wallsjust the way they are - I think there is no need to treat them specially. My favourite maps always have block walls (either block black or block white) which works because it draws the eye towards the interior spaces. I think your background is fine the way it is too.

The only thing I'm unsure about is the dark borders around your coffins. My laptop screen at work is unusually dark, so perhaps it looks fine on your screen, but to me it is hard to differentiate between the small 'island' of walling in the centre of the left room, and some of the coffins. For this reason, I would suggest you remove the dark border around all your objects.