The Mongols often destroyed entire cities, and most historians estimate the number they killed to be in the millions. Three, their empire didn't last. Within 80 years, they'd left China and been replaced by a new dynasty, the Ming, and in Persia they blended in so completely that by the 15th century they were totally unrecognizable. I mean, they'd even taken up agriculture. Agriculture, the last refuge of scoundrels who wanted to devote their lives to working instead of scoodily-pooping. They also weren't particularly interested in artistic patronage or architecture. I mean, your palace may last forever, but my yurt can go anywhere.