Only one wall remains of the Dickensian Marshalsea prison. There are some lovely photos of this on Wikipedia, including the one above: 'Angel Place, London. One of the walls of the Marshalsea prison can be seen on the right. To the left is Southwark's Local Studies Library at 211 Borough High Street, Southwark, London SE1.'
That man walks through the prison yard; separated only by a planck-length stretch of time.
The name 'Marshalsea' is a version marshalcy, "the office, rank, or position of a marshal" (deriving from Anglo-French mareschalcie). The prison was part of his court. But what gives the word unusual resonance, I think, is the way it ends: the counter-logic of 'sea', that vast open space, that opposite to a prison, working against the actual meaning of the word. The Northsea prison; the Caspiansea; the Marshalsea.