One of my favorite parts is the upgrade selection has correct and incorrect options. (in my opinion at least) For example, it takes experimentation to know how much to upgrade your attack to be able to two-shot certain enemies, and how much to upgrade it more when they randomly increase health. And also that speed is deceptively useful, even though it's just speed. If you really think about it, attack and speed are the only ones that help protect the windmill (as far as I'm aware), and for the other ones it's just figure out how many max hp increases you need until it's worth it to just set health to 100% when you're low.
Oh and I can't help but notice that my game is also an action themed enemy battler where you have to balance attacking enemies and repairing a central object, and if said central object runs out of health, you lose the game, and there's exactly three types of opposing entities trying to stop you that like to clone themselves to attack all at once. (Unless there's more than three in your game and I didn't play far enough in that case my bad)
Also did you pick windmill of all things for any reason (like you had the art, it would look cool at night, you wanted it farm-themed), or did you just decide windmill for fun>