Impressive that this was with your own engine!
Graphics:
simple but get the point across, some nice warcraft 1 nostalgia there. It's clear what each type of unit is, they are all clearly distinguishable from each other, it's intuitive that the enemies are evil and that they get progressively stronger etc. It was was clear what the bomb guys would do etc.
Audio:
music is decent for a compo entry, did you make it yourself? Audio suits the retro/low-res graphics like the difficulty level
UI:
Also simple but clear, no confusion anywhere. The text explaining that the game had to be restarted was a nice touch, of course a "retry" button would be better, but some text explaining it is much better than just leaving the player hanging.
Design:
You've "found the fun", balance feels right and it is a fun experience. I played quite a few times, couldn't get past wave 5 though :)
I like the level of difficulty. I like that the units move slowly, it forces one to be more strategic about their placement.
Impressive amount of enemy types and spells for a compo entry!
An option to place units directly from the shop (as opposed to first going into the inventory) would be nice, I don't think there will ever be a reason to keep units in the inventory. For spells it makes sense, though.
I was going to suggest unit health bars, but on second thought I quite like the minimalism without them.
It would be nice if there was some visual feedback on which level my units were, I suppose this would've been added if you had more time. Also, not sure if there is any info anywhere on what exactly improves with a level-up, but I think that would be nice.