Too hard, requires pause-scumming to proceed.
Graphics, sound, music, presentation overall: Excellent!
However there are too much flaws that bring down the game score considerably:
1- Abscence of checkpoints: It's very frustrating to die at the last wave (which is much longer and filled with capital-class ships btw) and having to restart.
2- Heavy clicking/managing on a fast pace: Strategy games are better off giving player TIME in order to actually strategize. Since this is a defacto "tower defense" game, then I can state clearly that the "breaks" the player get between waves is not enough.
3- The towers AI is bogus. They often keep firing on soon to die ships, allowing others to take cheap shots at them. Adding insult to injury: The shots go on forever (so their range are a LIE!) and won't damage any other enemy (in range of the tower or not).
I don't see how can this game be healthy to kids. I don't think most of them will ever reach the last wave.