The Fort de Bertheaume is a fort in Plougonvelin in the Finistère department in Brittany. The fort is located on a tidal island and it sits about 38m above sea level. The site has been used in Stone Age and in 1588 the English destroyed the first fort. In 1695 Vauban decided to build a new fort to guard the straits of Brest. Nowadays connects to the dry land via a footbridge.