I enjoyed the premise but found the balance left a bit to be desired. It would have been nice to have paced this out a little bit over the couple of 3/4 levels where it begins easy and only gives you the option of two units to summon. You could then introduce the other units in subsquent levels and increace the difficulty to match whilst show chasing which units counter which enemy turrets.
Overall a cool concept that would have been elevated by a touch of visual flare.