From Wye Waltz Wiki
Chepstow Port wall was built in the 13th century to protect the parts of Chepstow that were not surrounded by water. As a "port" wall, not just a mere wall, taxes and customs duties were levied on goods coming in and out of the town. Much of the portwall still exists and is a very impressive example of it's type. There were originally around a dozen large towers and entry was gained through the Town Gate.