It's all right, but there's one problem...
The game looks... okay, I suppose, though something seems rather off about the animation on the beavers that I can't really place, and the concept is pretty sound. I like how the upgrades are tied in to how skillfully you play; it's entirely conceivable that you'll be able to max yourself out a few waves before the boss fight as long as you do well and aim for the head as often as possible early on.
The main problem I have is with the difficulty curve, which is a mostly flat line, followed by a nigh-horizontal one right near the end.
The game, early on, is stupidly easy. This isn't a bad thing; considering that your upgrades are, again, entirely based on your own skill, it's a good idea to get the players eased into the system, and allow them a few, relatively 'gimme' upgrades.
But it never really picks up for a long time. As long as you do well in the first ten or so, rather easy waves, you'll have pretty much everything you need to make it to the boss; having all my stats maxed meant that, while there were definitely a few exciting moments- they were limited to the last two non-boss waves and were nonetheless easily repelled by one of the special powers I'd bought. I was able to make it to the boss fight, on my first try, without the beavers so much as touching the dam a single time.
...THEN we get to the boss fight. Quite simply put, the way this battle works, the player is not prepared in any way for it. Let's start with his attacks. Though you can knock him back when he decides 'to hell with you, I'm punching the damn dam', there's basically NO way you can deal enough damage to him to fend him off before he hits your dam at least once- particularly if you can't hit the pilot in the head because he decided to jump close to the top of the screen. Though it's hard to say whether I hate that attack more, or the one where he throws the rock with the mystery hitbox that takes FAR too many shots to destroy before it crashes into the dam. Top it off with the absurd amount of health you give him, as well as the facts that you are given NO notification that the game will disable all of your special powers for the battle, the fact that you're given so few opportunities to hit the weak spot that may not be entirely obvious to all, and last but not least, the lovely fact that I HAVE TO PLAY THROUGH THE ENTIRE DAMN THIRTY-MINUTE GAME AGAIN IF I LOSE TO THE BASTARD, all add up to a badly-designed boss fight that sucks SO much fun out of the game that it's hard to recommend the rest of it.
I mean, if I have to play through an entire game all over again, at LEAST give me more than three distinct enemy types so I don't get sicker still of looking at them the second time around.