a portcullis. The gatehouse is elliptical, and its western and eastern elevations show considerable differences. The western front has three machicolations over the gate with murder holes inset. The historians Oliver Creighton and Richard Higham note their "fine architectural detailing". The dating of the machicolations is uncertain, but comparison with similar, dated, examples, such as those at Cooling Castle in Kent, suggests a construction period in the 14th century. They cannot be original to the gate, as their positioning would have obstructed the portcullis. The eastern front is less decorated and displays evidence of more substantial reconstruction. It has a centrally placed round-headed window. The pedestrian arches through the bridge also differ in design, the northern being pointed and the southern having a flat "Caernarvon" head. Internally, the gate has a single room and a garderobe. The original internal access was by way of a stone spiral staircase, but this was subsequently destroyed and a wooden replacement was inserted. The room in the tower measures long and wide and has an attic and an 18th-century roof. The roof was reconstructed in 1832. From the upper room, the machine-cut rafters are visible. Drawings from the 18th and 19th centuries show a chimney in the roof, but the presence of a fireplace in the tower room is not mentioned in the available sources. The medieval roof was flat, with a castellated parapet and a wall-walk. In 1996, the bridge was included on a list of potential World Heritage Bridges by the UNESCO advisory body, the International Council on Monuments and Sites. The criteria for World Heritage status required that the bridge be of "outstanding universal value" and illustrate "a significant stage in bridge engineering or technological developments". The bridge is one of only two surviving fortified bridges in the United Kingdom, the other being in Warkworth, Northumberland. There, the gatehouse stands on land at one end of the bridge, rather than on the bridge itself, and is described by the archaeologist John Steane as "less impressive" than the "superb" Monnow Gate. A recent (2016) appreciation by the historian Richard Hayman describes the Monnow Bridge as "arguably the finest surviving medieval bridge in Britain". Such bridge towers were common across mainland Europe and, to a lesser extent, in Great Britain from medieval times. British examples included the Mardol Gate in Shrewsbury and Froome Bridge in Bristol. Continental examples include the Frias Bridge, near