The Citadel Town of Erbil, or Qala't Erbil, is an impressive elevated settlement that has been inhabited continuously for millennia. Although it's exact origins cannot be traced with certainty, there are strong archaeological indications and finds, however, to indicate that it goes back to some 6000 years. The fortified town itself must have been present during to the Assyrian Period (1365-612 BC), or some 3000 years ago.
Some archaeologists believe that the site has been settled in since the Neolithic to Mid Bronze Age period (6000-1500 BC).
There are several conjectural possibilities as to how Erbil Citadel Town originated and developed its present form. They may be outlined as follows:
1. Gradual Accumulation:
That the mound represents a gradual accumulation of historical settlement layers rising slowly to reach its present height of some 30 meters. If one assumes that the age of the citadel is around 6000 years then this means that its height has been rising at the rate of 1 meter every 200 years. Although this rate seems to be a very slow one but is, nevertheless, reasonable when compared with archaeological evidence.
2. Assyrian Settlement:
That it may have been an Assyrian settlement with a ziggurat in the middle surrounded by temples. And that when it was destroyed and abandoned, it turned into a heap of ruin. Then, at a later stage, it compacted and presented itself as a very useful "Tell" and defensible ground for human habitation.
3. Man-Made Mound:
That the mound was artificially created by people who desired to live in this fertile land but needed a fortified site. Of course, this would have needed a massive human effort which is unprecedented in the history of human settlements in Mesopotamia.
4. Natural Mound:
That the mound was a natural one, perhaps a few meters high and risen gradually by human habitation. The flat land geography of the area, however, makes this proposition unlikely. Also, the shape and the slope of the Tell appear so 'organized' that it makes it unlikely to have been a typically undulating natural mound.