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Porto earned its nickname -- Cidade Invicta, The Undefeated City -- through an 18-month siege during the Liberal Wars of 1832 to 1833. It is the kind of self-image that sticks. The city has always been a working place, exporting cork, cloth, and the fortified wine that took its name from this port and went everywhere. The locals are called Tripeiros, tripe-eaters, because when Prince Henry the Navigator provisioned his fleet for the 1415 assault on Ceuta, the good cuts of meat went to the sailors and the organ meats stayed behind. Porto people ate tripe and kept the name as a badge of honor.

What makes Porto worth walking is the layering -- Roman settlement, medieval cathedral, Baroque tower churches, azulejo-tiled train stations, modernist concert halls.

São Bento railway station has walls covered in blue-and-white tile panels depicting Portuguese history, and it is one of the more extraordinary commuter stations in Europe. The Clérigos Tower, designed by Nicolau Nasoni in the 18th century, anchors the skyline. The Dom Luís I bridge, completed in 1886, spans the Douro on two levels -- the lower for pedestrians and trams, the upper for the metro -- and the view from the top is the view that appears on every postcard. Across the river in Vila Nova de Gaia, the port wine lodges age their barrels in long low warehouses and offer tastings.