The town was founded in 1157 by Berchtold IV von Zähringen. Its name is derived from German frei (free) and Burg (fort). Its most ancient part is conveniently located on a former peninsula of the River Sarine, protected on three sides by steep cliffs. The easily defended city helped the Dukes of Zähringen to strengthen and extend their power in the Swiss plateau in the area between the Aar and the Saane/Sarine.Beginning at the time of its inception, Fribourg built a city-state; initially, the land it controlled lay some distance away. When the dukes of Zähringen died out in 1218, the city was transferred to the related Kyburg family. They granted the city its former privileges and wrote the municipal laws in the so-called Handfeste in 1249, in which the legal, institutional and economic organizations were established. Several treaties with neighbouring city-states, including Avenches (1239), Bern (1243), and Murten (Morat) (1245), were signed at this time.