The game functions well! However, I don't feel like I have much to do in the game other than just placing a new turret/block every now and then..
Some design suggestions would be to engage the player a bit more. Even if you're only using a build mechanic, just giving the players different turret options that affect enemies differently or giving the enemy dynamic movement can help stimulate creativity on the player's end.
That said, the game seems to have been built well and the visuals are very pleasing to look at! As simple as they seem.