I am surprised no one has mentioned what is most likely to be the actual answer: towers are closer to heaven. In medieval thinking, the ground is less prestigi

I am surprised no one has mentioned what is most likely to be the actual answer: towers are closer to heaven. In medieval thinking, the ground is less prestigious because it is closer to hell. “Lower” forms of life are called that based on the high-is-close-to-god metaphor. When wizards draw pentagrams on the ground they are doing darker magic that invokes the powers of Hell. So, a tower is like an insulator. Great and good magics will need to be invoked up in the air.