I really didn't like this one at first, but came back and played for a bit and it sort of grows on you. I would suggest adding a roll over window for the upgrade options and stats. It would make monitoring levels easier esp. when you have a line of fast moving enemies. I agree with not having the upgrade show if you can't afford them. A lot of thought went into this one, good job!
I think that you managed to make a really cute kids game that may not keep adults entertained for long, but I think a lot of kids are going to love this.
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "Protector"){kind=icon}
 "filler"){kind=icon}
 "Sling Junior"){kind=icon}