Monnow Bridge is the only medieval bridge left in all of Great Britain that still has its gate tower intact. The architectural and historical significance of the bridge and its tower have resulted in its being listed as a Grade I listed building in order to preserve it. Originally constructed starting in 1272, the bridge was built as a defense during medieval times, and was also used as a toll road to support public works. Ammunition was often stored within the upper level of the tower, which was used many times to fend off attacks.
The bridge and tower are made mostly of Old Red Sandstone, along with six other types of stone. In the original construction, the only entrance was through the main opening, which had a portcullis with pointed ends that dropped down to block unwelcome visitors. Nineteenth century renovations saw the breaking through of the two side passages for pedestrians and tourists.