I'm a winrar! :P There's one main thing that makes this game a little less enjoyable: no control of the camera. I think if you could just implement that, possibly using this tutorial from KidsCanCode, and get some better graphics and sound going, you'd have a great game!
https://kidscancode.org/godot_recipes/3.x/2d/touchscreen_camera/index.html - note that in this tutorial, he goes into the project settings and enables "Emulate touch from mouse," a useful tip for also making your game slightly more mobile-ready, where tower defense games are really popular. If you look at my game (this is not meant to get you to rate my game :P), I used the above tutorial to create the moving around the overall map.
Great try, would love to see more from you!
Edit: Something I forgot to mention is that this is an incredible submission for just one person working alone. I've tried to make tower defense games before and they are hard. Very impressive that you submitted this even with the very basic graphics and stuff!