Bern, also spelled Berne, city, capital of Switzerland and of Bern canton, in the west-central part of the country. It lies along a narrow loop of the Aare River. The existence of the ancient castle of Nydegg, guarding a crossing over the Aare, probably led Berthold V, duke of Zähringen, to found Bern in 1191 as a military post on the frontier between the German-speaking Alemanni and the French-speaking inhabitants of Burgundy. After the extinction of the Zähringen dynasty (1218), Bern became a free imperial city. Gradually it extended its power by acquiring surrounding territory, becoming an independent state that in 1353 entered the Swiss Confederation, which it soon began to lead. After a devastating fire ravaged the predominantly wood-built town in 1405, Bern was rebuilt with sandstone. Although much of the surrounding metropolis has since been modernized, the centre (Old Bern) still remains intact from that period.