The Grosse Horloge — the Big Clock — is the 14th-century stone gateway between the harbour and the old town. The original tower was the eastern gate in the city walls; in the 17th century it was crowned with the present clock and a fanciful belfry. The arch beneath is a tunnel about ten metres long, and you pass through it dozens of times in a visit to La Rochelle because everything funnels through here.
Look up as you go through and you will see the limestone vault still carved with the marks of generations of carters and porters. The tower frames the cleanest postcard view of the harbour — stand under the arch on the port side at sunset and look out across the basin.