The correct options are
A To keep out invaders.
B To watch out for potential threats.
D To act as a security checkpoint.
The Great Wall of China was built by the Chinese ruler Shi-Hwang-ti in the 3rd century BCE to prevent an invasion by the Tartars. It acted like a security checkpoint where the guards kept watch for any potential threats and attackers. It is 2900 kilometres long, 22 feet high and 20 feet wide with watch towers at regular intervals. It has 10,000 towers.