Very basic caves are generating seemingly-fine. They still need some more details, a few fixes here and there, enemies, indoor areas (probably the same cave ones I use in the forest tileset), and a boss room... but it's a start.
Additionally, as part of the cave stuff, I redesigned the way I build walls and other multi-tile structures out of necessity. I used this new code to improve how I generate trees for the forest tileset. They should no longer have random holes and bits missing.