Am finished
Well. This Ludum Dare was scheduled at exactly the wrong time - I was on vacation the week before (with no wifi, so I couldn't vote on themes) and I was flying on a plane on Saturday and didn't really get time to work.
So I decided to go the lazy way and use Unity and make a 3D game, which worked out pretty well.
By my best estimates, the total amount of time I spent on my game is under 12 hours (~3 hours on Friday, and ~8 hours today) so I wasn't really expecting to make anything great.
My "finished" product is a technically usable tower defense game. It is notably my first 3D Ludum Dare game, as well as my first one since Ludum Dare 33 to not have music. (Or any sound, for that matter).
Here's a screenshot that sort of gives some idea of what's going on:
So. It's okay. I hope next time goes better.