This is a “great stockade” built for military defense purposes in ancient times, just like small stockades. There two places guard two gates. The stockade gate is in between the cliffs, just like there are walls made of iron and steel, which are magnificent. The crack only allows one person at a time, so setting up a stockade gate here, makes it a good military base that fits the description of the old saying “one man guards the pass, even ten thousand enemies are unable to get through”.