The northwest corner of Beijing's ancient city wall forms an oblique angle rather than a right angle, primarily due to geographical influences during its Ming Dynasty reconstruction. The Yuan Dynasty Dadu city wall was originally rectangular, but when the Ming rebuilt it, the northwest corner bordered Jishuitan (ancient Gaoliang River), where marshy terrain caused unstable foundations. To address this, the wall was shifted southward, creating the oblique angle. While there is also the feng shui legend of "Heaven tilts to the northwest," geological factors remain the main cause—and this missing corner still exists today.