In 391 BC, Celts "who had their homes beyond the Alps streamed through the passes in great strength and seized the territory that lay between the Apennine Mountains and the Alps" according to Diodorus Siculus. The Po Valley and the rest of northern Italy (known to the Romans as Cisalpine Gaul) was inhabited by Celtic-speakers who founded cities such as Milan. Later the Roman army was routed at the battle of Allia and Rome was sacked in 390 BC by the Senones.