I like the idea of a tower defense inside of the human body! I couldn't quite tell if the turrets were themed as such, but I figure that's minor relative to the core mechanics.
The resizing of towers was interesting and I saw that it changed the usual tower defense style, where I had to plan for future turrets that I might want to place nearby. I do wish that the towers showed you their range (as a preview) before you placed them, since especially at first I had no idea how large a given tower's range was. But generally, pretty cool idea and execution!