Cherbourg's Grande Rade is not a single monument so much as a huge piece of maritime engineering. Its long breakwaters were started in the late 18th century and completed after decades of interruption, turning a vulnerable Channel anchorage into a major naval harbour.
You see it best from the waterfront, from the sea on a harbour trip, or from the hill by the Liberation Museum. The scale is the point: Cherbourg's story makes more sense once you understand how much effort France spent turning this coast into a protected port.