The Chinese name of the section of the road in the New Territories was later changed to Tsing Shan Kung Lo (青山公路; lit.

Speaking in the Legislative Council in 1909, Governor Frederick Lugard cited the facilitation of trade and police control as reasons for the road's construction.

After leaving New Kowloon, it goes uphill past Kau Wa Keng and Tai Ching Cheung along a four-lane expressway to Kwai Chung and downhill into Tsuen Wan.

Next, it goes along the south shore of the Western New Territories, via Yau Kom Tau, Ting Kau, Sham Tseng, Tsing Lung Tau, Tai Lam, Siu Lam and So Kwun Wat and then reaches the Tuen Mun New Town, which was also known as Castle Peak (after which the road was named).

It then passes through Mai Po, San Tin, Lok Ma Chau (near the Chinese border), Pak Shek Au and Kwu Tung before terminating at Fan Kam Road in Sheung Shui.