 Complete the following phrase:  Su Prefecture was established under the Sui and renamed Jiuquan Commandery under the Tang. Its seat was established just within the extreme northwest angle of the Great Wall near the Jade Gate. It sometimes served as the capital of the province of Gansu. Along with its role protecting trade along the Silk Road, Suzhou was the great center of the rhubarb trade. The old town was completely destroyed in the 