I noticed that in a lot of tilesets, the walls' ceiling tiles have this sort of "hollow" look to them, where you can see the edge of the tile, but the inside is blackness, giving a sort of pseudo-realistic, line-of-sight type view, and I really kinda liked that, so I thought I'd give that treatment to the RTP walls too. Some of the tiles, the cave walls especially, didn't come out quite as clean as I'd hoped, but they at least fit together correctly for the most part...
