Here are some pre-rendered isometric dungeon wall sprites.  Mix and match to create sprawling dungeon levels.

This is a PNG file with an alpha layer, created from Blender.  The blender file can be found in the 3D art section.

These tiles are purposed for a 64x32 base tile size.  Each tile here is 128x128 (the walls stand taller, and sometimes wider, than the base of the tile).   Example of it in use (actually a screenshot from a demo engine): http://pfunked.deviantart.com/art/Iso-Dungeon-Walls-Demo-136966447

I am working on a matching object (crates, statues, etc) and floor (stones, cracks, grates) set.  After that I may make sets with other themes.