The word Marshalsea is the same as 'Marshalcy', that is, the rank of a marshal in the army.
The name Marshalsea was applied to a prison in Southwark, London, on the banks of the Thames (shown with a blue dot on the above map). The prison was in existence from 1329 or earlier until it was closed in 1842. It was notorious as a debtors' prison where conditions, and life expectancy, varied depending on how much a prisoner could afford to pay. It was run privately for profit; the length of sentence depended largely on the whim of a prisoner's creditors. The father of Charles Dickens spent time in the Marshalsea for debt. It was also used to hold men awaiting sentence under court martial for crimes committed at sea and, during Elizabeth's reign, Catholics charged with sedition. It was where the infamous 'bloody' Bishop Bonner spent his last days.