A tower defence game where the vulnerable base moves is an intriguing concept, and I had a lot of fun with it here! I loved the opening cutscene as well - however, I think putting the turrets on such a long timer was a bit harsh - I'd often be kind of screwed if the based moved in an unfavourable position. Perhaps you could destroy one and you'd reduce the timer to be able to purchase another one? (So that you can't just place turrets wherever you want without cost)
Otherwise, great job!