The South Gate is the oldest (and for most of history, the only) point of entry into the citadel. The gate was an important part of the citadel defences, second only to the perimeter wall itself. The gate is at the lowest point of the citadel, but whether this was a cause of or effect on its placement is unknown. Some say it faces south to look out toward Mecca.
The current iteration is a reconstruction of the 19th century design.
It was typical to have just one gate, because such an opening is obviously the the most vulnerable point for external attack. It is extremely unlikely that the citadel had more than one gate. Yaqut al-Hamawi (1179-1228) mentioned in his Mujam alBuldan a northern gate at Erbil; however, this is almost certainly a reference to the lower walled town, not the citadel itself.
The gate was completely rebuilt in 1860 by usta (master mason) Ismail al-Sinini. Photographs show it had a huge pointed brick vault, two strong doors, a large courtyard, government serai (offices), a guest house, prison, clinic, shops and stables. This was demolished in 1960 due to critical structural concerns and replaced in 1979 with a modern reinterpretation in brick by Iraq Consult, an Iraqi architectural firm.