What was the original purpose of the Great Wall of China?

The Great Wall was built primarily as a defensive fortification to protect Chinese states from nomadic groups of the north, particularly Mongolian and Manchu tribes. The wall served as a barrier against invasions and raids. Options A and C are incorrect - walls hinder rather than facilitate travel, and pest control was never a function. Option D is incorrect as the wall was a practical military structure, not a religious monument.