A real marvel of modern engineering (19th century, to be exact), the Luís I Bridge in Porto is arguably Portugal’s most impressive bridge. Completed in 1886, the bridge was the longest double-deck bridge in the world at the time of construction, and while it has been surpassed by others it remains one of the most beautiful. Pedestrians take charge of the upper deck with stunning views of the Douro and Porto below, while general traffic motors along underneath. This is one of the great icons of Porto.