Southern Citadel (tr. Güney Kale), situated between the King's Gate and the Royal Citadel, is in the eastern part of Hattusa. A north-heading path leads to it from the King's Gate, and the best views of the fortress can be had from the Royal Citadel.

Several centuries after the collapse of the Hittite Empire, a large new settlement rose from the ruins of the ancient Hittite capital. It was created by the Phrygians and the Southern Citadel is their fort, erected near the end of the 7th century BCE, most probably replacing an earlier, older fortification. At roughly the same time, the Phrygian builders also rebuilt the former Hittite Royal Citadel (tr. Büyükkale), located to the north.

Both fortresses were capable of offering shelter to the people living nearby and their possessions, at least for a short period of time. This fact suggests that local landlords were concerned with the protection of their subjects and that the threats to their safety were serious.

The Southern Citadel was part of a fairly large Iron Age settlement that extended over part of the Lower Town. It is a formidable defensive structure: suffice it to say that the walls of its foundations are 4 metres wide.

The path leading up to the Southern Citadel passes the foundation walls of the Phrygian buildings and ends at in front of a Hittite cult chamber which was preserved under the Phrygian fortress wall. To the south-east of the citadel, there are the so-called Eastern Reservoirs (tr. Doğu Havuzlar) - a complex of cisterns that were intended to supply Hattusa with drinking water. So far, two reservoirs have been discovered, 2 meters deep and measuring about 60 by 90 meters. They were built on water-tight rock, and their bases were sealed with clay to prevent leaks. They were supplied with water from a nearby spring, and, additionally, clay pipes brought water from other sources located outside the city.

Two chambers were discovered to the north and west of the Eastern Reservoirs, which probably served religious functions. Both have stone vaults with very interesting construction technology. Chamber 1 (northern) is 3 meters high and 4 meters deep. It is in deplorable condition and only a small part of it has been reconstructed so far.

Much more intriguing is Chamber 2 (western), called the Hieroglyphic Chamber (tr. Hiyeroglifli Oda). This chamber was built into the northeast end of an artificial dam in front of the water cisterns. As the name suggests, many hieroglyphs were discovered there, most of which are legible to this day. Their excellent state of preservation is due to the insulating properties of the earth under which the chamber was located for thousands of years. The current state of Chamber 2 is due to intensive reconstruction work.

Chamber 2 is a room of religious worship. It was built on the orders of the last Hittite king, Suppiluliuma II, around 1200 BCE. The ruler himself is visible in a bas-relief on the left wall of the chamber, depicted as a warrior with a bow, spear, and sword. His name is written in Luwian hieroglyphs above a portrait.

The back wall of the chamber depicts an unnamed deity wearing a long cloak, with a winged sun disc above his head. He carries a sign similar to the Egyptian ankh (i.e. key of life) in his hand.

The right of the chamber wall features six lines of text. It describes Suppiluliuma II's invasions and conquests and well as foundations. The king mentions that with the support of various deities he had captured many lands, including the Land of Tarhuntassa. He also built many new cities and offered sacrifices to the gods of various lands.

The last sentence mentions the construction of a "divine earth road", which may mean that the chamber itself was a symbolic passage to the underworld of the dead. According to expert scholars, this sentence emphasizes the function and the construction purpose of the chamber. It was intended to state that there is a passageway to the underground, and thus the chamber is the entrance to the world beneath the earth.