The Monnow Bridge is the only surviving medieval bridge with the gatetower on the bridge span in the UK and is therefore, now, unique!
The bridge was built in the late 12th century and stood as a means to cross the River Monnow into Monmoth town, but in the 14th century the gatehouse, called Monnow Gate, was added as a defensive structure to help protect the town at this vital crossing point. It would not have looked quite as it does today though as the bridge itself was narrower and the three prominent arches above the main pass-through arch were added at a later date, and there was also a portcullis that you would pass under to gain access to the town (the grooves for it are still visible).
After it was built the structure served as a toll house as well as a defensive structure, but it has also been used as a guard room, gaol and dwelling over time.
Due to congestion a new bridge to hold a two-way road had been proposed in 1981 but it took until 2004 for construction to be completed. It is situated only 0.1 miles downstream (you may well have come over it to get here!)
Up until 1858, the gatehouse was the scene of annual muntlings (battles, or fights), between rival gangs from the main town of Monmouth (Uptowners) and people from Overmonnow, also known as Cappers' Town (where Monmouth Caps were traditionally made). These took place on the 1st and 29th of May, and the fighters (mainly youths) arming themselves with besoms (think of the traditional witches broom) amoungst other weapons (I'm thinking early mods and rockers!).
Worth a visit whilst you are here is the impressive Millenium Plinth just on the western side of the bridge, which has 40 ceramic tiles, each showing different Monmouth events from the Iron Age up to Modern Day.
So to the cache. This cache is in an unusual place and is in a high muggle spot, but you shouldn't look too dodgy as other people look here too (you'll see)! 
I can't take credit for the 'outer container'!
Oh, and please make sure you hook it back on round the corner so that others looking don't see it.