What was built between the great wall and nanking

The City Wall of Nanjing (Chinese: 南京城墙; pinyin: Nánjīng chéngqiáng) was designed by Emperor Zhu Yuanzhang (r. 1328–1398) after he founded the Ming Dynasty (1368–1644) and established Nanjing as the capital 600 years ago. To consolidate his sovereignty and keep out invaders, he adopted the suggestions of advisor Zhu Sheng to build a higher city wall, to collect grains and to postpone the coronation. Then, he started to build the city wall. It took 21 years to complete, and used 200,000 laborers to move 7 million cubic metres of earth. The City Wall of Nanjing was among the largest city walls ever constructed in China.[1][2] The enclosed Nanjing City is about 55 square kilometers.

Their solution was to tax the colonists.