The last and shortest of these open stretches was practically in Southwark, seven-eighths of the way across. On the far end of this gap the carriageway was over-arched by a stone castle, of ancient-looking design, but only about three hundred years old. It was the highest structure on the Bridge, for it served both as watch-tower and as choke-point. It dated to an æra when military operations were of a more straightforward character, so that a bloke on the top of the tower, looking south to discover Frenchmen or Saracens coming up in force, could sound the alarm and slam the doors to the Bridge. It was called the Great Stone Gate.