Defense Game? Ch-YEAH!
I love these kinds of games... Although this one was a bit awful. The "Clicks to defeat" thing wasn't a good choice, since you'll have around 3-5 enemies at your door at the same time, so you'll always lose at least 4 health every few seconds. Also, the upgrade system is a bit humdrum. Having to switch between a new weapon and a defense boost just to survive, that doesn't appeal too much to me. For a defense game, this is one I'd prefer to pass over... Tweak some aspects, and it could actually be pretty good.