Just to get warmed up, I (mostly) finished my cave tile set.
Here's a half-size edition, with a superimposed grid so you can see where the tiles are:
This should account for every combination of wall and floor. The final tiles are 160x160. The art I made was 1600x1600. The spec is that the art should be made three times the final size, to enable future HD editions of the game. By making the initial art ten times the final size, I ensure that even the HD art is shrunk.
As the good book says, shrinking your art covers over a multitude of sins. The downside is my geriatric computer starts to choke whenever I load the original files.
I've made tile sets before, but only pixel art, so I had to devise the process for hand-drawn art on the fly.
First, because I wanted the cave to look like it had been carved out by water, I made a 3D model of a cave wall with simplified, cartoony, flowy features, as my initial attempts to draw it by hand failed to capture the look I wanted.
Using this as reference I sketched out one side of the wall. Krita is a free digital painting program that allows you to easily create tiling drawings as you go.
I added red to the shadows, especially at the bottom, to help subtly communicate the location of the walls to the player, so he can more easily take it in at a glance. Similarly, I made the floor tile a little on the blue side so that it theoretically recedes beneath the characters.
By scaling, flipping, erasing, and painting together, this turned into the horizontal and vertical walls, which, along with the floor, form the core of the tile set:
By setting these two tiles, one over the other, and deleting parts of each, I formed a wall cross, and a block.
Every other tile is simply a matter of copping and pasting parts of those five tiles together and blending the seams between the parts.