The information box says that the Han added 4,000 miles of wall. Refer to the mileage scale. This can be true because the Han Dynasty lasted 400 years. During this long time, the Han Dynasty was convinced that the best way to keep its enemies out of the Chinese territory was to build a large wall. So they did. They constructed a wall frame of barely 20' high, 30' long, and 25' wide. And they used that frame to build the wall. Let's not forget that the Great Wall of China was constructed for 2,500 years, so it was continued by other Chinese dynasties.