This was quite fun and enjoyable! I really like how the upgrades are represented visually and not merely as stat changes - like how skeletons get bigger or better armed. At some points it got really hectic with what looked like many explosions (though that could be units being revived as demons?) and overwhelming armies of skeletons and/or demons.
Only issue I found was that at some point I stopped generating money altogether, and neither myself nor the demons ever seemed to be defeated despite taking too much damage - I suspect this was more of a technical issue?
Though that was minor as it came near the end. Overall: great stuff!