The current citadel dates back to the reign of Louis XIV, but Saint-Martin was already fortified in the Middle Ages: in 735, the duke of Aquitaine Eudes raised a fortress here, and also a monastery and a church dedicated to Saint Martin of Tours. The little city was governed by several lords, such as Mauléon, Thouars, or La Trémoille. The first harbour was put up in 1537; in 1627, Saint-Martin was besieged: La Rochelle was a Protestant city, so was the little island of Ré.
So, 8000 English soldiers led by duke of Buckingham, invaded the island's tip. The governor, Toiras, took shelter in the Saint-Martin's fortress. But the English besieged it during four months! The French resisted and Buckingham surrendered...
In 1682, the new governor Pierre Amou re-fortified the citadel: they called the famous military architect, Vauban. A book says they raised the new stronghold within 65 days! Nowadays, we can see the beautiful gates of Toiras and Campani, but also the Governor's house.