The Ile Harbour is situated to the west of the town of St Malo, on the far side of the main entrance to the port. This meant it was an excellent position for a fort to protect the town from naval attacks. Situated on the east side of the entrance to the port was the Fort du Petit Bé, whose fire overlapped with that of Fort Harbour to ensure no enemy vessel could enter the harbour.
There had been a redoubt on the island previously, but it had fallen into ruin by Vauban's time. In the climate of the 1690s, when the threat of an English attack on St Malo was very real, it was replaced by a much more substantial fort. The new work was built of stone and contained barrack buildings, stores and a chapel. Vauban recommended, as he did for other island forts, that the commander should be a retired or invalided naval officer, who would be able to understand the tides and interpre the movements of hostile vessels. For most of its life the fort mounted between 10 and 20 guns, all facing out over the harbour entrance.
Unique among the island forts designed by Vauban and Siméon Garangeau for St Malo, Fort Harbour has a bastioned trace on its seaward side. The reason for this is probably that the fort was vulnerable to a land attack from the front at low tide, so it needed proper flanking cover. The rear however, was protected by a steep cliff, so a bastioned trace was not necessary on that side. Fort Harbour played no direct role in the English bombardments of St Malo in 1693 and 1695, because the attacks came from the north and not from the west.
Visiting Fort Harbour
Unfortunately Fort Harbour is privately owned and cannot be visited. However it is sometimes possible to walk to it at low tide and it can be seen from the walls of St Malo and the Fort du Petit Bé.