In terms of user experience, I think it’s alright, but it lacks something like a minimap, or a “path indicator”, after finishing waves I got lost a good number of times, and didn’t understand where to go. As for gameplay, maybe rename waves into some kind of parts? like 3 level parts consisting of 3 waves each. And maybe each level part could give an upgrade. Also you could look into game design difficulty curves, where not exactly each wave should be harder, and, for example, getting an upgrade right before “easing” level of difficulty could lead to getting more powerful feeling and enjoyment overall.
