I'm glad you said this wasn't serious. One level and one tower with no changes? One tower with some control, such as targeting or upgrades could be interesting.
Because of the four entry points and speed at which gems are generated, this seems more like a click-fest than a strategy or puzzle game to me. I had a lot more success that way.
If that's what you wanted, good job, otherwise, slow it down some (in my opinion).
Nice fundamental idea.
 "Cosmic Crush"){kind=icon}
 "TowerDefense: Back to basics"){kind=icon}
 "Diamond Fever"){kind=icon}