 The main port, Saint-Martin, was fortified by Vauban in 1681 as a component of the belt of forts and citadels built to protect the military harbour of Rochefort.It was later used as a depot for convicts on their way to the penal settlements of New Caledonia and French Guiana.Prisoners included Alfred Dreyfus, en route to the penal colony of Devil's Island after his conviction for treason.