Medievel China's geography is very diverse land including deserts, mountains and fertile river basins. China's rivers also play a major role in China, used both for transportation and for irrigation. Much of the northern wheat fields and southern rice fields are irrigated, and transported from rivers. Let us not forget the great wall of china built in the Ming dynasty. it was used to protect from invaders from the north. Troops would patrol the wall to defend China against attack. Once over 6,000 miles long, the wall is now about 3,750 miles long.