The Great Wall of China is some 6,259.6 km (3,889.5 miles) long. It was started in the 5th century BC to protect the northern borders, Beijing and the Imperial Ming Tombs from nomadic groups. Built in sections up to the 16th century, there was significant rebuilding and refurbishment during the Ming Dynasty (1368 - 1644), the best preserved sections are from this period. Many sections of the wall on the plains are made of compacted earth rather than stone as this was all that was available. Across the mountains, stone was used extensively though there are additionally some trenches and the use of natural barriers provided by rivers and mountains too. This section is at Mutianyu (慕田峪) some 70km (43.5miles) northeast of Beijing in Huairou County. Here the wall, built of granite, is between 7 and 8m high (23 - 26ft) and between 4 and 5m wide (13 to 16ft). There are 22 watchtowers on the 2.25km (1.4mile) Mutianyu section of the wall. A cable car has been built to take visitors from the car park to just below the wall, though a pathway is still available for those with more stamina. The view from the wall is spectacular, looking over large sections of the wall, the watch towers, mountains and on a clear day, the skyscrapers of Beijing can be made out on the far horizion. The Chinese characters "Study Chairman Mao" has been made in stones on the mountain side below the wall 1.4km (0.8mile) to the northeast of the cable car station, the characters are 200m wide by 31m high (656ft x 101ft).