I used a a wacom tablet and photoshop. Interesting tidbit: I painted this scene 100% with the same photoshop brush: a "chalk" brush that is in the default brushes set, with opacity jitter set to pen pressure in the "transfer" options.
1. Set up
I've put down some messy lines just to figure out what kind of scene I am going for. The perspective is all off, but I'm not worrying about that at this stage (perhaps I should be?). I'm just getting down the general idea.
2. Blocking In
Now I've taken a big, soft-ish brush and slapped down a couple colors on a new layer, one for the basic shape, and another to define where the light is coming from in the scene (mainly from above here).
I slapped down a quick background in a layer below the walls.
At this point I'm defining the shapes of things without the original lines. Here's where a lot of the perspective issues are getting fixed (hopefully).
Adding more stuff to the scene! Keeping the direction of the light source in mind and staying loose and relaxed.
Adding texture, cracks, and more colors to increase the realism.
7. Details-Pass 1
More texture, more colors, more cracks...
8. Details-Pass 2
Here I've continued adding details, such as patches of moss here and there. I've also defined the puddles better, added a piece of foreground rubble, and added a big light shaft streaming in through the broken roof. I also have Aura in there to double check that the scaling and colors work together. It's pretty much done at this point!
9. Final Adjustments
All I've done here is fiddle with the levels, exposure, etc. a bit to make it a little more dramatic. And it's done! Here are some close-ups of the finished scene:
And that's it!