Many of these forts later transformed into large Siberian cities .
From the 17th century , after the start of the Russian conquest of Siberia , the word ostrog was used to designate the forts founded in Siberia by Russian explorers .
Many of these forts later transformed into large Siberia cities .
From the 17th century , after the start of the Russian conquest of Siberia , the word ostrog was used to designate the forts founded in Siberia by Russian explorers .