There stand 14 major passes (Guan, in Chinese) at places of strategic importance along the Great Wall, the most important being Shanghaiguan and Jiayuguan.

The world’s longest man-made structure stretches over 6, 300 miles (10, 139 kilometers) from Shanhaiguan in the east to Lop Nur in the west.

Please google 'jiayuguan fort' have a look at the actual building and compare with attached image to get an idea what I am looking for.

JIAYUGUAN, China (Reuters) - A full solar eclipse will sweep across the Arctic and Siberia before ending in western China, where it will kick off the month in which Beijing hosts the Olympic Games.

Planeloads of cheerful foreign eclipse chasers converged on Jiayuguan, in Gansu Province, and in the hot deserts of Xinjiang, to watch the sky go dark and a halo wreathe the hidden sun.