A like the game! the concept
What I felt was that there comes a time when enemies will hit you no matter what. It would be nice to have some kind of balance where you can take on all enemies, either because you've significantly upgraded your attack power and speed to do so. Because after a certain level, you can only rely on your shield to survive, not your weapons.
I feel like there's perhaps little space on the screen from the moment your enemy appears and reaches your castle. Maybe add some kind of zoom where you can see beyond your castle, and you can further extend your attack range?
Or maybe I would play more based on how long the enemy resists and not on their supersonic speed, because from the moment the enemy appears at the edge of the screen until they reach the castle, you don't even have a second to react, and it kind of frustrates you that you can't kill them.
It could also be some kind of skill tree, like allowing you to shoot more than one arrow at a time, or even more if you keep upgrading, and some other type of attack and upgrades. Because I feel like the only attack you have is one arrow, and on the other side, five or more enemies are coming at you at once.
Maybe add some boss fights too every 10 levels?
It would also be nice to see the enemies' energy bar, even if it's just a small detail below, to know how much damage you're doing to them. To know how much to upgrade your attack power past a certain level.
Just some ideas! Keep going!