We cast off from Douro Marina as the afternoon softens into gold. The Douro is the reason Porto exists, a river of wine, granite and light. And from the water you meet the city the way it was always meant to be seen.

The first great arch overhead, completed in 1963 to a design by Edgar Cardoso. At the time the longest concrete arch span in the world. On the way back to the sea, its curve catches the last of the sun.

The long granite façade on the Porto bank is the city's 19th-century customs house, now a riverside congress and culture venue. A reminder of the days when everything that entered Porto came up this river.

High on the Porto hillside, these terraced 19th-century gardens look straight down onto the water. The original glass palace is long gone, but the green slopes and the panorama over the Douro remain one of the city's finest views.

The Vila Nova de Gaia waterfront, lined with the port wine lodges where the wine has aged for centuries. As the lights begin to warm, the rooftops and the old rabelo boats make the postcard everyone knows.

Across the water, Porto's UNESCO-listed old town tumbles down to the river in a tangle of medieval lanes and tiled houses, the Church of São Francisco rising behind. The heart of the city, seen from below.

Porto's great double-deck iron arch, completed in 1886 by Téophile Seyrig, Gustave Eiffel's former partner. Passing beneath it is the moment most people came for. And at dusk it begins to glow against the sky.

your turning point upriver: the elegant iron railway bridge opened in 1877, designed by Gustave Eiffel's firm and the oldest of Porto's great spans. Retired now, but still one of the most graceful shapes on the river.

The finale. We drift back downriver to the mouth of the Douro, where the river meets the Atlantic and the palm-lined gardens of Passeio Alegre open onto the sea. This is where the sun goes down. The whole sky over the ocean. And the reason the cruise is timed the way it is.

We head back in the blue hour, the bridges now lit and the city glittering on both banks, to step ashore where the evening began.