Some annoying bugs
When the game ends, most of the annoying things happen:
When the scoreboard appears (a bit too fast btw), clicking it away sometimes causes the game reload again. Not nice if player must wait a long time in order to play again.
When you are back in main menu, old music stays in the background mixing with current music, also that warning siren, in case you lost.
I passed all waves, and I got score 8. What kind of score is that?
"Towers" might be a bit unbalanced, I passed the game using only that 1k costing starship, without upgrading. High damage/long range ships let too many enemies pass.
I was thinking, "what an idea, tower defense game with moving towers. What next, static enemies?" Unfortunately I was right. I placed a couple of ships too close the entrance, and suddenly enemy stopped. It was destroyed, but no new enemies appeared, and closest ships of the entrance/enemy kept on firing that location. They did not move even I ordered them to. I made radical decision and sold them, and when I sold the last firing ship, enemies started appearing again (only a few, then came the next wave).
And about the next wave: I can't move the ships when that next wave thing appears. Fortunately I can scroll the map, which is annoying, why do it have to be so big?
I did not find pause in game. Also, when I increased game speed, enemies started to stack a bit, some of them even turned back, since it was too crowded ahead.
Anyway, enemy AI is good, and music is very good.