Fort Louis sits on a low promontory west of the town centre, built by Cardinal Richelieu in 1625 as the siege fortification from which the royal forces ultimately strangled La Rochelle into submission. The fort preceded the great 1627–28 siege — Richelieu first established military control of the landward approaches, then built the famous dyke across the harbour mouth to cut off the fleet of English ships that came to resupply the defenders.
The fortification is relatively modest today — what remains is substantial stonework rather than a preserved castle — but the site commands a view of the harbour and the surrounding landscape that makes its military logic immediately clear. Alexandre Dumas used the siege as the central dramatic set-piece in The Three Musketeers, and historians of the Wars of Religion still consider it one of the defining episodes in the destruction of French Protestantism.