Oldie but a goodie
The game at first was challenging, but once I was able to have 1020 tower guards and 67 repairmen, and maxed out both auto defense turrets, the game went into auto-pilot. I was able to go off and work on something else, and only stop to continue from the upgrade screen. I still got hte com hub, but the airstrikes are useless. Since there was no end to the game, I exited the game.
The game will hold your interest for about 30 to 40 minutes before you can go into auto-pilot mode (ie meaning you no longer need to shoot).
Graphics aren't too bad for this old of a game either. The sequel was also great and more challenging than this of course.