This text is a fragment of a guidebook to Hattusa: "The Secrets of Hattusa".

The Turkish word Yerkapı, meaning the gate in the ground, is derived from the postern that leads out through the rampart below its highest point, below the Sphinx Gate. This name quite accurately captures the essence of this part of Hattusa fortifications. So far, archaeologists have discovered twelve similar tunnels under the fortifications of Hattusa. However, Yerkapı is the best-preserved structure of them all. It is also the only postern of Hattusa that is still accessible. This artificial 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 probably played a representative role, not the defensive one, although historians do not agree on this topic.

The tunnel in the Yerkapı embankment is 71 meters long but just over three metres high. It has a clear slope towards the outside with an inclination of 15 degrees. The tunnel was built using corbel vault technique, with massive boulders, and then covered with an artificial embankment. 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 corbelling 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 well over three thousand years. The tunnel and its outer exit are well-preserved. Unfortunately, the details adorning the inner exit have been destroyed. The external façade of this mighty structure was covered with limestone blocks.

The floor of the tunnel was covered with white screed, which reflected the little light that entered. On the inside, below the Sphinx Gate, the entrance is integrated into the steep slope, as the exit was built into the ramp there as a massive structure. Both sides were once closable with double-leaf doors. Nothing can be said about the function of the passage, but it was certainly not a sally port that would not have been built so clearly visible.

From the outside, the wall is about 30 meters high and was covered with stone paving over a width of 250 meters. Vertical grooves were worked into the paving at intervals of 21 meters to drain rainwater. Large areas of the stones are missing today; they were used in the later phase of the city for the second, forward city wall on the top of the wall. At the two ends, in the west and in the east, stairs lead to the top of the wall, which is why the wall gives the impression of a truncated pyramid from a distance.

A military function for the mighty wall can be ruled out. The clearly visible exit of the postern and the stairs on both sides of the ramp speak against this. The 35-degree gradient would also have been an easy obstacle to overcome for a trained warrior. Peter Neve and his successors as excavators of Hattusa, Seeher and Schachner, considered the wall to be a possible stage for ceremonial and cultic performances. In addition, there is the impressive effect of the monumental building on delegations arriving from the south. From the north, the complex, as the crowning glory of the city, could be seen from a distance of 20 kilometres, which in Hittite times was more than a day's journey.

In August 2022, Bülent Genç, Associate Professor at Mardin Artuklu University and the member of the Hattusa excavation team, discovered numerous hieroglyphs in the tunnel, painted on the stones with reddish-brown paint. A detailed follow-up investigation identified 249 Anatolian hieroglyphs. These are not inscriptions, but individual graffiti, most of which probably represent the names of people or gods. While some of the markings are too worn to be read, most are well-preserved and clearly legible.

According to investigations by the philologists involved, Metin Alparslan from Istanbul University and Massimiliano Marazzi from the Suor Orsola Benincasa University of Naples, at least eight groups of characters were identified, and one character alone was counted 38 times. Such painted hieroglyphs are hardly known to date; they have probably only survived for so long due to the special, uniform climate inside the postern. According to the excavation director, Andreas Schachner, with the newly found hieroglyphs, the scientists will have the opportunity to better understand what the tunnel was used for.