Okay, so reading through this thread, I had a few ideas that might help...

It looks like Rage of Aquarius's your man, so there's not much to say there besides advising that you use the tileset in original and unique ways. Moss isn't really a factor of the set, but it can still be (limitedly) replicated by using vines or grass bridges, purely as eyecandy (or within walls, where they can't mess up your gameplay). Also, try placing a translucent black tile in Layer 6, then build some platforms and "castley" stuff in Layer 7. That way, the Layer 7 castle "ruins" will be darker than the Layer 4 castle "ruins", and the prospective Jazzer won't be confused by what appears to be side paths, but is really simple eyecandy. A mid to bright green Layer 8 fade color background might work well, although this is more up to you.

Also, have you thought about what song you're going to use? Because if you're going for the ruined/wet castle look, I would highly recommend dark_ages.xm, available on Modarchive. I was going to build a level similar to the one you're building (Ironically, also with RoA), but gave up on it due to other projects I was working on at the time. Dark Ages was the song I had chosen for it, and I think it'd work pretty well for you. But again, it's up to you.

Hope this longwinded spiel helps!