Castle Cary's cylindrical "lock-up" was built in 1779 at a height of 10ft (3m) and width of 7ft (2.1m), according to a plaque on its stone wall.
It is one of only six in the country and the only one that can hold wedding ceremonies, the town council said.
"Couples looking to inject an element of humour in their special day will revel in the 'dramatic irony' this building provides," it added.
The Round House has two iron grills for ventilation and was once used to incarcerate miscreants and drunkards.
"The bride and groom are able to make their vows under lock and keyâ€¦ and manacles if necessary!" the town council said.
Source: www.gov.uk
A tiny historical prison in Somerset has been licensed for weddings.