The Abbey of Saint-Michel-en-l'Herm dates from 682 AD when it was constructed by Benedictine monks from Noirmoutier. It was destroyed by Vikings in 877 AD. The new church was completed in 1047.
In 1569, Protestants attacked it, killing approximately 200 monks. The motives for the attack are still debated, but one theory is that the attackers sought treasure and manuscripts hidden by the Bishop of Luçon.
The final destruction took place during the French Revolution when the abbey was confiscated and most of buildings destroyed.
Carlsten is a stone fortress built on the orders of King Carl X of Sweden following the Treaty of Roskilde, 1658 to protect the newly acquired province of Bohuslän from hostile attacks. The site of Marstrand was chosen because of its location and its access to an ice free port. Initially a square stone tower was constructed, but by 1680 it was reconstructed and replaced by a round shaped tower. Successive additions to the fortress were carried out, by the inmates sentenced to hard labour, until 1860 when it was reported finished. The fortress was decommissioned as a permanent defense installation in 1882, but remained in military use until the early 1990s.
The fortress was attacked and sieged twice falling into enemy hands. In 1677 it was conquered by Ulrik Frederick Gyldenløve, the Danish military commander in Norway and in 1719 by the Norwegian Vice-Admiral Tordenskjold. At both occasions the fortress was returned to Swedish control through negotiations and treaties.