Watch your step

According to Felipe Fernández-Armesto's review of Genghis Khan: life, death and resurrection by John Man (via Arts & Letters Daily), Genghis Khan supposedly made the streets of Beijing "greasy with the fat of the slain". But

In the long run, however, he was a constructive destroyer. His empire , at his death in 1227, spanned Eurasia, creating havens of peace around the silk roads and steppelands. Accelerated contacts enriched Eurasian civilisations. Technologies that trans- formed Europe's future - gunpowder, the blast furnace, paper money - arrived in the West. Traditions of scientific empiricism, dormant in Europe since antiquity, revived as Westerners began to share attitudes to nature formerly confined to China. Italian merchants, French craftsmen and Franciscan missionaries met in the depths of the Gobi.

Italics mine. I think the biographer has fallen a little too much in love with his subject.