Port Wine Cellars of Gaia

Porto's defining industry lives, confusingly, in the other city: Vila Nova de Gaia, across the Douro, where the great port houses have kept their lodges since the 1700s because the north-facing riverbank stays cooler. The barcos rabelos moored along the quay — the flat-bottomed boats that once ran barrels down from the Douro rapids — now float as billboards for the names painted on their sails: Sandeman, Graham's, Cálem, Taylor's, Ferreira. Behind them, whole hillsides of warehouse hold literal decades of wine sleeping in oak.

A cellar tour is the mandatory Porto experience and genuinely worth it once: the cool dark halls of 550-litre pipes, the smell of evaporating "angels' share," and a working lesson in what port actually is (fortified mid-ferment with grape spirit, hence the sweetness) and the crucial family tree — ruby aged in bottle, tawny oxidising in wood for 10/20/40 years, vintage declared only in great years. The tasting at the end converts more sceptics than any wine course.

Choosing a house: Cálem by the bridge is the polished high-volume introduction (good museum wing, optional fado-show evenings); Taylor's up the hill is the connoisseur's tour with the best terrace view in Gaia; Graham's rewards the walk west with seated tastings above the river; the small houses (Poças, Kopke) trade spectacle for intimacy. Book a timed tour online in season — English slots fill — and taste tawnies against rubies to learn your own preference fast.

Make the afternoon of it: walk over the Dom Luís I bridge's lower deck, tour and taste, then ride the Gaia cable car (or climb) to the Jardim do Morro for sunset — the whole of Porto's stacked façades going gold across the river is the region's best free show, and slightly better after two glasses of 10-year tawny.