Has some gameplay issues
The game was fine & worked great, but there were several problems.
1. The flying monsters were too strong & too fast, there is only 1 weapon that works well against them & its the most expensive, and it has really expensive upgrades as well. They need to be turned down a bit, or the anti air weapon needs to be cheaper.
2. The green slimes, I has lots of problems with these monsters because they will often 'teleport' through your guns. Which can be a very big problem. Also when you kill them and they split into more, only the first one gives you money the 'clones' dont give you a thing.
3. Another problem I encountered was lag, I have a 2.4 ghz dual core system, and was still getting a fair amount of lag. Maybe you should include a download version =).
Also after beating the 6 levels there really isnt much more to do. Maybe add a survival mode (where you have a map, and see how may round you can go), or add a sandbox mode (where you have alot of money).