Monnow Bridge Tower is a rare 14th century stone gate tower, founded by Henry of Lancaster. Defended on the Welsh side of the River Monnow, by the Clawdd Ddu rampart and ditch and originally built with a single arched passage, it fortified the late 13th century bridge. In the 19th century, the gatehouse was pierced for pedestrian use and a hipped roof added to the upper storey. Sadly it is now the only remaining example in Britain, where the gate actually stands on a medieval river bridge. At the top of Castle Hill, stands Monmouth Castle and 14 miles south-west is Usk Castle.