The Douro curves through Porto in a 900-metre gorge whose slopes carry eight centuries of wine-trade infrastructure — stone quays, barrel warehouses, granite steps worn concave by stevedores. Six bridges cross that gorge, and five were designed by engineers trained under Gustave Eiffel. The oldest, Ponte das Barcas, was a floating pontoon that collapsed under fleeing troops in 1809; the youngest, Ponte da Arrábida, held the world's longest concrete arch span for twelve years after its 1963 opening. Between them, the 1886 Maria Pia railway viaduct demonstrated wrought-iron cantilever principles later applied to the Forth Bridge, and the two-tier Dom Luís I carries pedestrians above, metro trains below, and 4,500 tonnes of riveted steel across 395 metres of air. River traffic on the Douro predates Roman occupation — amphorae shards from Lusitanian trade vessels still surface after winter floods. By the 18th century, flat-bottomed rabelo boats carried port wine casks downstream from Régua to Vila Nova de Gaia's lodges, their square sails rigged to catch valley thermals. That commercial traffic ended with railway construction, but the rabelo hull — shallow draft, high prow, centre-board keel — proved ideal for tourist sailings once the waterfront's UNESCO designation in 1996 catalysed restoration of Cais da Ribeira's medieval arcades. Today's 6 bridges cruise porto tickets tours depart from those same quays, tracing a route that threads beneath every span and offers sight-lines impossible from the clifftop neighbourhoods of Massarelos and Fontainhas. The standard 50-minute sailing begins at Ribeira, passes beneath Luís I's lower deck, turns at Freixo's cable-stayed towers four kilometres east, and returns through the industrial ironwork of Infante and São João. Sunset departures add port wine service and extended commentary on the Arrábida's parabolic geometry. Evening light renders the south bank's terracotta roofscape in ochre and sienna, while the north bank's schist cliffs hold late sun until 19:30 in summer months. The cruise operates year-round; winter sailings between November and February offer clearer air and fewer passengers, though cabin heating remains minimal and upper decks stay exposed to Atlantic weather systems that funnel upriver with little warning.