Decent as a beta, perhaps.

The game lacks a lot of polish. For example, if I pause, then go to the upgrades screen, then back to the build screen, the game unpauses. I can unpause during the splash screen between levels. If I "win" in sandbox mode, all the upgrades I purchased will carry over to the next game I play, even if it's not in sandbox mode (didn't try in real mode).

A few other suggestions:
+Hot Keys
+Rather than having to click and drag cannoneers/archers, I should just be able to click that section of the wall to purchase them. Alternatively, hot keys.
+Healing of some sort. Purchasing ground units just didn't seem worth it. A PacMan is never capable of killing $350 worth of enemies, even if you put all your money into upgrading them.
+Attack indication. If an attack makes contact, how about a little blood splatter, or some other visual indicator?

That's all I could think of from the polishing front. All in all, a decent game for a game with no depth.

A good game

This is a good game, it's very enjoyable and playable. My only beef with it is that there's a memory leak hiding somewhere because the game got slower, and slower, and slower, and slower until it was crawling. Otherwise, good job.