The forbidding fortress above is Broadmoor Mental Hospital, where the most dangerous criminals in Britain are kept. Every man in there is not only a murderer, but he's also clinically insane. There are only 250 or so inmates, but it takes a facility this big to handle them securely.
Above, the hospital walls.
The four surrounding towns all have sirens mounted on towers to give warning if an inmate escapes. The sirens are turned on at a fixed time every week for two minutes to be sure they're in good working order. School children are drilled in what to do if the siren goes off any other time.
This (above) was one resident of Broadmoor: Graham Young, a serial poisoner. Prevented by his incarceration from murdering more family members, he devoted himself to poisoning other inmates. He learned how to do it from medical books he found in the hospital library. He figured he had to experiment on other inmates so he'd do a good job on the general public when he was released....which he was after only nine years. Horrors ensued.
Here's another Broadmoor guest: Robert Maudsley, called "Britain's Hannibal Lecter." Killing a truck driver got him sent to Broadmoor where, like Graham Young, he turned his attention to other inmates. He killed several. In one instance he forced another patient into his cell, and he...he....Aaaargh! It's too horrible to talk about. Suffice it to say that he was nicknamed "The Spoon" after that.
Now they keep him in a glass cage, something like the one that housed Lecter in "Silence of the Lambs." His furniture has to be made of compressed cardboard. When he's taken out to exercise he's accompanied by six guards. He's reputed to have attempted to bite his mother's face when she came to visit him.
Had enough? Me too! This is giving me the creeps!