There is nowhere to hide in this prison as tall walls were the view from ever window.  Unless Mary could find an appeal there was no way out.  However, if you had the right key this unlocks a complex underground maze of tunnels under Oxford Prison.  There is a direct underground path to link the prison to the Court in St Aldates.  For this reason, up until the 1980’s very high profile prisoners facing a trial have been held at Oxford gaol including one of the Great Train robbers.