Good work! Here's some quick feedback from my first impression, but do note I didn't get to see nearly all there is:
- A button to reset the save would be appreciated, especially for experimenting.
- My ADHD-wrecked brain demands a speedup slider.
- Consider increasing base health. You can absorb, on average, 10 attacks. So if 4 units arrive, your base is toast even if you kill 1 enemy per attack cycle.
- By making units retarget when new enemies arrive, you can also protect the base a bit and make its HP a strategic resource you can reliably use rather than merely a buffer in case 1 or 2 enemies slip through.
- Consider having fewer levels with more radical changes and instead different difficulties for each level to have the same amount of effective levels. That way progression feels more noticeable.
- High tier units need some buffs. Having 2 units is effectively quadruple the strength to 2. HP get multiplied by more than 2 (since enemies might waste damage) and attack by a bit less than 2 (since attack drops when the first of the two is killed). Meaning 2 boars are just better than a bear, and a rat is just a cheaper boar.
- Meteor Storm is significantly superior to the other two spells, killing 2, 3, or even 4 waves of enemies. And those who don't die are so low, they stand no chance against your units. The other 2 spells beat 1 or 2 waves with heavy support from your own units.
- Being able to cast your spell only once per level, on top of only a limited number of enemies spawns coming at a very steady, unvaried pace gives the game less of a strategy and more of a puzzle feel.