Choose the best answer.
The Great Wall of China was built more than 2300 years ago to protect China from invaders and it runs along what was once the northern border of the country. The wall is more than 2400km long and 8m high. It was wide enough for the top to be used as a road. Towers were built into the wall and soldiers were kept there to guard the country. In the 13th century, however, invaders from Mongolia crossed the wall and conquered most of China. Today the Great Wall of China is the only thing built by humans that can be seen from satellites in space.
People were able to use the top of the wall ... .