The word Portugal derives from the Roman-Celtic place name Portus Cale; a city where present-day Vila Nova de Gaia now stands, at the mouth of the River Douro in the north of what is now Portugal. The name of the city is from the Latin word for port or harbour, ''portus'', but the second element of ''Portus Cale'' is less clear. The mainstream explanation for the name is that it is an ethnonym derived from the Castro people, also known as the Callaeci, Gallaeci or Gallaecia, who occupied the north-west of the Iberian Peninsula. The names ''Cale'' and ''Callaici'' are the origin of today's ''Gaia'' and ''Galicia''.