It's ok
It's not bad, but it does have a few issues. Being limited to where you can place your towers is one, especially since some towers have very little range. The biggest issue is the random targeting. Towers will shoot something until something else comes along, then maybe they'll switch back and forth, or they'll just shoot at the second target, leaving some bugs to get by just because they're not as close as other bugs, even though they're still within range. Being able to upgrade towers would be a nice touch as well.