Holy crap I am absolutely amazed. The polish you have achieved for this game is simply amazing jethro, great job! The cinematic at the beginning was an impressive surprise! The mechanics you have implemented are very tight, and there were no obvious bugs I encountered. It didn't take long for me to figure out the controls and game flow, and by my third attempt I had managed to dominate the play area with my units.
In terms of improvements that could be made I noted the following points.
1. The first thing I noticed (right from the moment the cinematic started playing) was that the game as a whole is very dark. The lighting probably needs a bit more work, including the cinematic - it wasn't until nearly the end of the cinematic that I noticed the skull was actually animated!
2. The cinematic would have been a lot better with some audio.
3. In terms of gameplay once you had the spawn area covered it is impossible to lose the game. A system with multiple spawns that selects the most vulnerable spawn location could be a good potential solution to this. You could also block off the enemy spawns the same way as you have blocked off the tile in front of the cauldron.
4. Again in regards to the gameplay there needs to be some kind of win condition so that there feels like there is some sort of reward/achievement.
5. I would recommend capping the upgrades to the units so that they cannot be made OP. Some tweaks to the upgrade values are probably a good idea too.
6. When the player loses the game there should be a bit more warning before the game resets so that the player is not wondering what happened.
7. In terms of the art although the humanoid model is a good minimalist model it does not suit the overall theme of the game. It was also hard to distinguish between the different types of units but this may also be largely due to the poor lighting.
Overall I think this is an incredible game with a lot of potential to be made into something you can publish. I am simply amazed at how polished the mechanics are considering that 3 days ago a prototype for the game didn't even exist. Great work Jethro!