Ashur (also spelled Assur or Asur) is built on a rocky hill rising forty meters over the west bank of the Tigris, near where it is joined by the Lesser Zab.
Ashur was founded almost 5,000 years, long before the ancient Assyrian Empire. It began as the trading center for a merchant network that would bring in goods from the north (in Asia Minor, modern-day Turkey) to exchange with the south (modern-day southern Iraq). A ruler called Ušpia living in the 3rd millennium BC was credited by later residents of the city with the construction of the earliest version of the large temple complex there dedicated to Ashur. The city – as cities in ancient Mesopotamia were commonly identified with their patron deities – was a representation of Ashur. The unique rocky hill and the city founded upon it were the god Ashur himself, and the temple within was the god's only shrine.
Texts found at Ashur and elsewhere describe how it was periodically integrated into larger cities to the south – Akkad, Ur, and Babylon, for example. However, it maintained a unique cultural identity. Distinctively, the local rulers of Ashur did not describe themselves as kings. This would later change under the reign of Ashur-ubalit, a transformation described as the the changing nature of kingship in Assyria, and Ashur became the seat of a kingdom governing the land between Ashur, NIneveh, and Erbil – between the Tigris and the Zagros. From this time, the Assyrian state would expand and take other lands. To justify their conquest of lands under the patronage of other gods, the Assyrians would request their subject make economic contributions to the worship of Ashur.
In 879 BC, the capital of Assyria moved from Ashur to Kalhu. The capital of Assyria would never return to Ashur. However, it remained religiously significant. The shrine was cared for by residents of Ashur until 240 AD when it was destroyed during the Sassanian conquest of Mesopotamia.
Ashur is located near the modern Iraqi city of Tikrit, it is known today as قلعة شرقاط (Castle Sherqat) in Arabic. It is commonly transliterated as Kaleh-Shergat or Qala't Shargat. Excavations began by Germans under W. Andrea's leadership in 1913-1914. When looking at artifacts, a winged sun-disk (sometimes with the upper body of a beared god-figure) represents Ashur.
Ishtar Temple built by Tukulti-Ninurta. 1243-1207 BC. Assur. Lead. Istanbul Museum of the Ancient Orient. Inv No 8856, 8857. Image my L. M. Clancy, 2009/08/28
Great article from when Ashur was nominated to be a world heritage site