Vila Nova de Gaia, or simply Gaia is a city and a municipality in Porto District in Norte Region, Portugal. It is located south of the city of Porto on the other side of the Douro River. The city proper had a population of 178,255 in 2001. The municipality has an area of 168.46 km². and a total population of 302,295 inhabitants (2011), making it the most populous municipality in Norte Region. Gaia together with Porto and 12 other municipalities makes up the commonly designated Porto Metropolitan Area.
It is also well known for its cellars (locally known as "caves") where the world famous Port wine is stored and aged. These cellars have become a major tourist attraction.
Vila Nova de Gaia already existed under the Roman Empire as the city of Cale. It developed most likely from a preexistent Celtic Castro, or Neolithic settlement. The origin of the name Cale (or Gale, since in Classical Latin there was not always a clear distinction between the letters "g" and "c") is likely Celtic, from the root "Gall-" with which Celts referred to themselves, similarly to Galicia, Gaul or Galway. The river itself has a presumably Celtic name, Douro (Durus in Latin) maybe after Celtic "dwr", water. In Roman times the bulk of the population probably lived south of the river Douro, and on the north side, there was a smaller settlement around the deep water port which now is the Ribeira neighbourhood of Porto. The name of Porto, in high medieval times Portus Cale, thus stood for the harbour ("portus" in Latin) of the city of Cale. With the important trade of the river Douro, which is easily navigable up to the Regua deep inland, the Porto part of the city came to overgrow Cale, and became the most important part of town, where the Bishopric and the merchants were established.
With the Moorish invasions in the eighth century C.E., the de facto frontier between the Islamic state and the Christian one came to rest for a considerable period of time on the river Douro, around the year 1000. With the constant raids and counter-raids, the town of Cale, or Gaia, was deserted and most of its inhabitants took refuge in Porto.
After the conquest and pacification of the southern side of the Douro river after 1035, with the exodus or expulsion of the Muslim populations, leaving behind semi-abandoned fertile lands, colonists from the north settled in exchange for better feudal contracts with the newly appointed lords. These migrants refounded the old city of Cale or Gaia under the name of Vila Nova de Gaia (new town of Gaia) around the old castle and ruins of "old Gaia".
The name of the double city of Porto and Gaia was frequently referred to in contemporary documents as "villa de Portucale", and the county of the Kingdom of Leon which was centered around it was named Condado Portucalense. This county was eventually expanded and after gaining its independence, became the Kingdom of Portugal.