The Russian conquest of Siberia began in July 1580 when some 540 Cossacks under Yermak Timofeyevich invaded the territory of the Voguls, subjects to Küçüm, the Khan of Siberia. They were accompanied by some Lithuanian and German mercenaries and prisoners of war. Throughout 1581, this force traversed the territory known as Yugra and subdued Vogul and Ostyak towns. In order to subjugate the natives and collect yasak (fur tribute), a series of winter outposts (zimovie) and forts (ostrogs) were built at the confluences of major rivers and streams and important portages. Following the khan's death and the dissolution of any organised Siberian resistance, the Russians advanced first towards Lake Baikal and then the Sea of Okhotsk and the Amur River. However, when they first reached the Chinese border they encountered people that were equipped with artillery pieces and here they halted.