Very nicely done! I knew I was having fun the moment I saw that you could upgrade each unit - I love stuff like that. However, I feel that being restricted to one upgrade for one unit per round makes it feel like enemies seem to ramp up in strength much faster the player. I'd much prefer if you got multiple upgrade points per round with stronger enemies (or some other similar balance lever against the player) - that way the player may feel more inclined to focus on spreading their upgrade points across multiple units, rather than just one. Sound and visuals were very charming, and I enjoyed the simple premise. Good work!