This extraordinary building in St.Ouen overlooking the sea is La Caumine a Mary Best. Built in 1765 to replace a guardhouse and magazine destroyed by fire. It was originally attached to a redoubt (or battery) and is the oldest defence in St.Ouen's Bay in Jersey. (There are lots of others but they date from the early 1940's and were built by the Germans using slave labour.)
Mary Ann Best was the daughter of an English soldier and a local woman. It's not known why the building is named after her.
It's painted white as a navigational marker.