Question

The ________, a barrier that linked earlier walls across China's northern frontier, was built to stop nomads from invading China. What should be in the blank space ( the line)

Answer 1

I think the phrase to be filled into the gaps is "the great wall", and more specifically, "The Great Wall of China". 

The earlier structures were were built as early as 7th century BC, and they were joined around 3rd century BCE.