Very polished! Love how you run faster while holding the turret - like in real life. :P Could be actually interesting I think, if the turret would still fire while moving it around, just much slower. Also I was kind of sad, that the upgrades did not carry over between levels. Would have loved ending up with the turret you showcase in the credits, haha! Lastly, take a look at this game, if you haven't already: it's also about moving around "only one" weapon for base defense and though it doesn't look as fancy as yours, I has some clever mechanics, that might inspire you for this game as well.