The Great Wall is one of the greatest feats of human engineering. My wife and I hiked the Jankou section and I consider it to be one of the most memorable experiences of my life.
This is a view from the doorway of the Oxhorn Tower. The stair case and subsequent door below is how the soldiers accessed the Wall from their homes. Zhengbeilou Tower, which I have posted on in the past, is the tower you see in the distance.