I love tower defense games, and I really like what you've done with the mechanics! The towers malfunctioning sold me on the story that everything's gone haywire (and let me focus on my ship). Cool game!
Also, I think it's very engaging that some of the enemies leave the track. In most tower defense games, it gets repetitive with enemies staying on the track, but it felt like a paradigm shift when that first little bugger flew towards my ship off the track! I felt way too safe (so to speak) up until that point. Great idea!