Such a cool concept! I love the mix of tower defense and action, and the Atari-style graphics and sounds give it a great vibe. It would be nice to be able to rotate tiles while holding them--I kept trying to do that, and getting them pointed the right way was a bit of a challenge. As far as balance goes, I think tiles should be able to take more than one hit. As it is, there's no incentive to build up a big machine, because you'll spend all your time running to replace parts. I pretty quickly settled into dropping a cannon or two right next to the main gear, and focusing my efforts on shooting manually, because trying to build out was too frustrating.
Great job on the boss design--they all felt different, and I loved the third boss picking up my tiles and throwing them at me. It was a lot of fun to play, and I'd love to see where you go with it if you keep developing.