original: During its heyday, Chang'an was one of the largest and most populous cities in the world.
simplifications: During its high point, Chang'an was one of the largest cities with the most people. Chang'an used to be of the largest cities in the world. Chang'an was a large and highly populated city. Chang'an was once one of the largest and most populous cities in the world. During its high point, Chang'an was one of the largest and most populate cities in the world. During its best, Chang'an was one of the largest and busiest cities in the world. During its peak, Chang'an was one of the largest cities in the world. Chang'an was a large and populated city during it's high point. Chang'an was one of the largest and most population cities in the world during its heyday. During its prime , Chang'an was one of the largest and most populous cities in the world.