So far, I’ve got 4 rooms of 3 prisoners:

It’s built almost entirely out of bedrock and iron. Quite inescapable, I’m afraid.

This is the medium security area. I haven’t built solitary and general population yet.

I’d drop in there, but I don’t want to rile the prisoners up with a surprise inspection. Suffice it to say there’s a loo, a bath-ish, and enough food and water for three days.

All four blocks share an exercise yard. I might build the solitary and general population wings off of it, just to be neat about it.

Prisoners are warned not to dig in the yard. If you do…

…it’s quite as inescapable as the standard cells, and you’ll be stuck down there for the guards to either let you out, kill you, or leave you in place as they please.