During the Neolithic period, the region was inhabited by Pre-Celtic tribes, who built religious and funerary monuments, megaliths, dolmens and menhirs, which still survive in areas on the periphery of Lisbon. The Indo-European Celts invaded in the 1st millennium BC, mixing with the Pre-Indo-European population, thus giving rise to Celtic-speaking local tribes such as the Cempsi or Sefes.