The walls in the cell are covered in lava. The prisoner respawns up through the cobblestone regenerating pilar below them. Entry can be done via the ceiling.
Outside Defence
The prison is surrounded in 8 mob bans, which have a AoE of 96x96 blocks. This includes a lot of Hi_There's lava grid to try to stop withers. There is also a mining detector above the prison.
More Security
There is an elder guardian box to grant Mining Fatigue 3. Guards can almost not leave their guard area, due to it regenerating with cobblestone
