Imperia is the fourth largest town in Liguria. It became a municipality in 1923 as a consequence of the union of Oneglia and Porto Maurizio, two towns on the Western Riviera divided by the mouth of the River Impero. The two nuclei remain distinct despite the administrative merger and extensive property development. Porto Maurizio was an ally of Genoa from the thirteenth century onwards, while Oneglia, the natural sea outlet of the Impero Valley, was allied with the House of Savoy from 1576. Oneglia's origins as a town date back to mediaeval times. Worth visiting are Palazzo Doria (now completely transformed), which was the birthplace of Andrea Doria, and the church of San Giovanni Battista. Porto Maurizio, which stands on a promontory, began to develop after 1000 AD: the oldest district is called Parasio. The cathedral and the historic buildings are well worth visiting.