Don't complain, build more towers!
The gameplay is simple although highly customizable. In essence, you determine what style and strategy you want to employ on a map. You may decide to use good old-fashion lead (turrets) or long range mortars to thin out the masses. You might even decide to shoot "lasers" at the bugs until they explode. The choice is yours, but luckily, every choice is satisfying.
The premise is to find the right defensive position on the map and build your defenses as wave after wave of alien bugs try to swarm your mining operation. The story is 100% Sci-Fi and should appeal to fans of Aliens and even the fans of the cult-classic, Starship Troopers. What's not to love about destroying rabid alien bugs?
I do have one bone to pick, and it's not with the game itself, but of the countless reviews, including Gamespot's review, that the tower AI is stupid. The AI IS NOT STUPID. Reviewers are quick to blame the AI when bugs are swarming and the towers target other targets instead of the one the player would like. The truth is these turrets are automated. When you place an automated turret in a position, it is going to shoot everything that moves, in other words, whatever comes across it's sensors is going to get blasted. Every automated turret in the game world works this way. You need only play a few levels of Half Life 2 to see this.
Thus, instead of blaming the AI for not shooting the target we would like shot, we should realize that automated turrets are simply designed to shoot anything that moves period. The towers are not going to prioritize targets. Therefore my advice is this, if you find yourself in this predicament where your turret is shooting but isn't' shooting the "right" target, you should relocate your tower or buy more towers! Because this problem of target choice only exists when your towers are being overwhelmed, you should seek to build more towers or place the towers in such a manner where you are getting any bugs that happen to squeak through. Create cross fire situations and back up your turrets with some artillery and this should solve your problem. Use strategy people. The turrets do the shooting, the player needs to do the thinking!
In conclusion, this is not a design flaw, this requires the player to be more thoughtful and realize when the towers can't keep up with so many bugs. This requires the player to react and account for the bugs that are getting by the towers. This game is awesome and whatsmore, I only paid 10 bucks. That is a steal for a game that has so much replay value!