It was very simple really, wine was grown in the Douro Valley and transported in casks by sail boats down to Porto at the river mouth which was a fine deep water port. it was loaded on to sailing ships for shipment to England.
The casks were off loaded and stored awaiting shipment, so the stone warehouses were built to store and protect it, it was then discovered that the older stuff was better, this started the whole technique of aging and blending.

The ships transporting, loading and off loading

The Port Houses

Plenty to explore, my favourites were Offley and Calem, best to go on line and book a tour, ideally in the morning, then follow with a lunch nearby.

My discovery, white port, a very good aperitif