The Great Wall of China is not one single wall but a series of walls built by different rulers over many centuries. The earliest sections are more than two thousand years old. Its main purpose was to protect the northern borders from attack, and soldiers lived in towers along it to watch for enemies and to send signals with smoke or fire. The best-known parts were built during the Ming dynasty, using stone and brick. Some people say that the wall can be seen from space, but astronauts have said that it is very hard to see with the naked eye. Today the wall is a famous tourist place, and millions of people visit each year. Some sections have been damaged by weather and by visitors, so workers are repairing them.

Question: The word 'series' in the passage means