See https://en.wikipedia.org/wiki/Wild_Fields https://en.wikipedia.org/wiki/Wild_Fields Basically the entire southeastern half of modern day Ukraine, which is

See https://en.wikipedia.org/wiki/Wild_Fields https://en.wikipedia.org/wiki/Wild_Fields Basically the entire southeastern half of modern day Ukraine, which is among the best cultivatable land in the world, was uninhabitable because it was naturally steppe and living there put you at constant risk of raids from horse nomads (Crimean Tartars). And because it was uninhabited, regular armies could not cross it (due to lack of forage), so the Tartars could not be atta Ked on a large scale.