As the article makes mo attempt at an explanation, here's one:
A unique medieval relic, High Bridge spans the River Witham near the bottom of High Street. The bridge is the oldest bridge in England that still has houses built upon it. The others are Pulteney Bridge in Bath and Frome Bridge in Somerset. High Bridge was built around 1160 to replace an earlier ford across the Witham.
In the Tudor period, houses were built upon the western side of the bridge. The narrow opening created by the arch of High Bridge has been called the 'Glory Hole'; its small, crooked arch sets a limit on the size of boats which can successfully navigate the Witham.