This is an interesting concept, a bit like tower defense but where you get to move the tower you're defending upwards (and you can control one turret at a time for additional defense). My main recommendation to make the game more fun would be to add tricky decisions the player needs to make. One possibility would be turrets with strengths and weaknesses against different enemy types, and/or occasional waves of enemies that can't be defeated quickly enough without manually controlling some turrets (and possibly swapping between turrets to get the different enemy types).