In accordance with the general usage of his times, Herodotus included under Assyria the whole of Mesopotamia, both Assyria proper in the north and Southern Mesopotamia.
His history would therefore have been of extraordinary value, and since nothing escaped his observant eye and well-trained mind, the religious customs of the country would have come in for their full share of attention.

As it is, we have only a few notices about Babylonia and Assyria, incidental to his history of Persia.[7] Of these, the majority are purely historical, chief among which is an epitome of the country's past--a curious medley of fact and legend--and the famous account of the capture of Babylon by Cyrus.

Fortunately, however, there are four notices that treat of the religion of the inhabitants: the first, a description of an eight-storied tower, surmounted by a temple sacred to the god Bel; a second furnishing a rather detailed account of another temple, also sacred to Bel, and situated in the same precinct of the city of Babylon; a third notice speaks, though with provoking brevity, of the funeral customs of the Babylonians; while in a fourth he describes the rites connected with the worship of the chief goddess of the Babylonians, which impress Herodotus, who failed to appreciate their mystic significance, as shameful.

We have no reason to believe that Ctesias' account of the Assyrian monarchy, under which he, like Herodotus, included Babylonia, contained any reference to the religion at all.

What he says about Babylonia and Assyria served merely as an introduction to Persian history--the real purpose of his work--and the few fragments known chiefly through Diodorus and Eusebius, deal altogether with the succession of dynasties.