A well made project, definitely something to be proud of for your second project. If you want to improve, I would focus on a few things. There currently is no background music during the gameplay, which is relatively easy compared to how much it adds. Focus more on veriety in game play, because right now there is a lot of effort focused on upgrades that only tweak numbers, instead of giving different experiences with those numbers. A big one is that the game now has a lot of wait time. The last 15 seconds of a round nothing spawns, then there is a delay between ringing sound, then text and then menu. I think that is something to improve. Also it might be worth looking into how to make a web version of your next project. For me it was surprisingly easy. And you don't have to develop for web, Unity can export it as a web game after developing it for desktop first.