You don’t put a prisoner in an ivory tower.
You put them under the ground, where they can’t see the sky.
Preferably, you don’t give them any natural light at all.
There’s a part of the story soon after the prisoner escapes — usually at night — when they realize they have made it.
They look up, see the stars, and weep.
It’s almost impossible to imprison a human that can see the stars.
But once you take them away, it’s only a matter of time before they forget how free they were under that big glistening dome.
And then you don’t need any walls to keep them prisoner!