Porto (or Oporto) and the nearby Duoro Valley is the true home of port. It was first developed here when British winemakers added a neutral grape spirit to stop the wine fermenting further on its long trip to the markets of England. This is what gives it that distinctive sweetness. Only Duoro Valley wines are true ports so this is the place to sample them.
The port houses of Porto are where all the wine used to be aged, brought down river from the summer heat of the Duoro Valley which would have caused it to age too quickly and instead aged in the cool coastal cellars of Vila Nova de Gaia. The finished port was then shipped away to England. These days with better cooling and insulation techniques the whole aging process can happen at the vineyard so these port houses are now more for storage of finished wines, some still in the huge casks, some already bottled. They are also popular for tours and cellar-door tastings.
Photo courtesy of Jaan-Cornelius K via Flickr.