Really good idea for a game! It's simple and entertaining!
I know that 3 days aren't much, but it could be better!
Some ideas:
-add some new upgrades and maybe change some already existing (+1 health isn't anything helpful)
-Maybe less ghosts? or is it that after like 20 seconds you have to just run away and plant turrets anywhere before you get surrounded
-some more touhou wouldn't hurt too
-some FPS issues were mentioned earlier, but it's propably easy to fix
Anyways, that's a solid game. Keep it up!