Nineveh was an old cult centre long before it was an imperial capital. Ishtar of Nineveh was distinguished from Ishtar of Arbela and had her own temple, priesthood and reputation, and her statue was important enough that a king of Mitanni sent it to Egypt in the fourteenth century BCE, apparently as a healing measure for the pharaoh. Prophetic oracles addressed to Assyrian kings in the name of Ishtar survive on tablets, spoken by named men and women attached to her temples, promising protection and demanding loyalty in return.
Sennacherib made the city his capital around 700 BCE, ringing it with a wall of some twelve kilometres, laying out gardens and bringing water from the hills by an aqueduct whose stone channel still stands at Jerwan. His grandson Ashurbanipal assembled in his palaces the collection that has shaped everything known about Mesopotamian religion: tens of thousands of tablets and fragments, gathered by agents sent to copy the holdings of temples across Babylonia, covering omens, incantations, rituals, medicine, lexical lists and literature.
Among them was the standard version of the Epic of Gilgamesh. When George Smith read the flood tablet in the British Museum in 1872 and found a Mesopotamian account of a man told to build a boat, of birds released to test for land, and of a sacrifice on a mountain, the discovery was reported as front page news and changed how the Bible was studied. Nineveh fell to a coalition of Medes and Babylonians in 612 BCE, and the burning of the palaces baked the tablets and preserved them.