Crimean Tatars and Nogai Tatars traditionally used the sparsely inhabited area of the Wild Fields on the south border of Russia immediately south of Severia to launch annual raids into the Russian territories along the Muravsky Trail and Izyum Trail. In 1591, a Tatar raid reached the Moscow region, forcing the Russian government to build new forts of Belgorod and Oskol in 1593, Yelets in 1592, Kromy in 1595, Kursk in 1597, Tsarev-Borisov and Valuyki in 1600. Tsarev-Borisov, named after Tsar Boris I, was the oldest settlement in Sloboda Ukraine. During those raids regions near Ryazan and along Oka River suffered the most. With the Russian territorial expansion south and east into the lands of modern Sloboda Ukraine and mid Volga River the conflict intensified. Sometime between 1580s and 1640s the "Belgorod Defense Line" with number of fortifications, moats, and forts was constructed in Sloboda Ukraine, providing security to the region. After a number of Russo-Crimean Wars, the Russian monarchs started to encourage the settlement of the region by Cossacks who acted as a sort of frontier guard force against the raids of the Tatars.