I survived with a score of 3880! Woo! Fun game
I also agree that this could be something more... hmm... multiplayer of course WOULD be nice, but I think single player could be made more interesting.
The thing I like most about the original "Defend your Castle" is the idea of the upgrades you can purchase and keep. Perhaps introduce your own unique way of spending points earned on upgrades such as faster fire rate? Perhaps an auto-fire sentry or denser walls or a shield that you can shoot out of but blocks projectiles coming in. All sorts of good stuff
I like to buy and keep upgrades
But as of right now, it kept me entertained through the whole game, so you have a great start!