This is a lot of fun to play! My only note is that the rounds seem to generate the enemies randomly causing difficulty spikes in some of the earlier rounds on some playthroughs. Once I was I made it to around round 20, I felt pretty unstoppable and had no problem clearing any rounds after that even if I opted not to upgrade for awhile. Maybe changing which enemies can spawn in the early (maybe first 4) rounds would help the player to be able to build some money to purchase upgrades. Another possible change would be to let the player collect the money during the round instead of after the round and purchase upgrades in the during the round. Even if the player had to click on the coins to collect them, this would add some additional engagement from the player during the round and help balance out the difficulty. To balance out the later rounds, maybe each round could scale the health of the enemy types up creating the need to keep upgrading. Overall, this was really fun and I hope you continue devlopement on it!