Isker

thumb|Sibier, the region and the city, can be seen on this map by Gerhard Mercator (published 1595), positioned on a left tributary of the [[Ob River. Mercator places Sibir correctly at about 58° northern latitude, but somewhat too much to the west.]]

Qashliq, Isker, or Sibir ( Qïšlïq) was a medieval (14th–16th century) Siberian Tatar fortress, in the 16th century the capital of the Khanate of Sibir, located on the right bank of the Irtysh River at its confluence with the Sibirka rivulet, some 17 km from the modern city of Tobolsk. The fortress is first mentioned in Russian sources of the 14th century.