The Turkish word Yerkapı, meaning 'the gate in the ground,' quite accurately captures the essence of this part of Hattusa fortifications. It is located inside an artificial embankment that forms the southern tip of the city walls. That embankment is 15 meters high, 250 meters long, and 80 meters wide at its base. Above it, there are city walls, with the access to the city provided by the Sphinx Gate.

Yerkapı tunnel is 70 meters long. It was built with huge boulders and then covered with an artificial embankment. The external façade of this mighty structure was covered with limestone blocks. The tunnel runs across the embankment, and its inclination is 15 degrees. So far, archaeologists have discovered twelve similar tunnels under the fortifications of Hattusa. However, Yerkapı is the best-preserved structure of them all. Yerkapı probably played a representative role, not the defensive one, although historians do not agree on this topic.

The impressive construction of Yerkapı was completed at the beginning of the 14th century BCE. It is based on the corbelled vault as the Hittites could not build true arches. Instead, they applied the method that uses the architectural technique of corbeling to span a space in a structure. The application of this method does not belittle the power of architectural thought of the Hittites. Their tunnel is still passable, after more than three thousand years. The tunnel and its outer exit are well-preserved. Unfortunately, the details adorning the inner exit have been destroyed.

The tunnel is located at the highest point of the city, so it is possible to enjoy magnificent views of the entire Hattusa and the surrounding plains. These plains were cultivated in the Hittite times, providing the capital city with food. During the wars, the farmers working in these fields could seek refuge within Hattusa city walls. After walking through Yerkapı outside the city walls, it is possible to climb the stairs to the embankment and get back into Hattusa by passing through the Sphinx Gate.