Everyone else has covered my thoughts pretty well. Enemies had good variety - they were visually distinct, and had clear behaviour differences.
The amount of income you get is pretty well tuned to always (usually) allow you to buy at least one thing per day, but you do have to make choices.
I think I understood the upgrades, but could have been wrong. (Health, passive speed, active speed, damage, arrows per minute - with potential other options including duration of active sprint, and travel speed for arrows.)
It's a shame your "border" is just the default unity skybox as this isn't part of your colour palette. Next time either set up some plainly coloured tiles, or set the skybox colour to match one of your 4 colours in the camera settings.