For thousand of years the culture of China has survived to form one of the most enduring civilizations in World History. One of the many reasons for this success has been the barriers to large invading armies that surround China. These can be divided into two groups, major natural barriers and man made barriers.
Seas- Japan, Yellow, East China South China
Mountains- Himalayas, Altan
Rivers- Huang, Yangtze
Jungles of South East Asia
Deserts- Gobi, Taklimakan
The Great Wall and the Grand Canal
Barriers are just that, difficult terrain for an enemy army to cross. Determined armies can succeed in crossing them but at such a cost in men and materials that can be deemed to high for all but the most determined foes.