Concerning the sites of other cities in Northern Babylonia, considerable uncertainty still exists. The extensive mounds of Tell Ibrâhîm, situated about four hours to the north-east of Hilla, are probably to be identified with Cutha, the centre of the cult of Nergal, but the mound of 'Akarkûf, which may be seen from so great a distance on the road between Baghdad and Falûja, probably covers a temple and city of the Kassite period. Both the cities of Kish and Opis, which figure so prominently in the early history of the relations between Sumer and Akkad, were, until quite recently, thought to be situated close to one another on the Tigris. That Opis lay on that river and not on the Euphrates is clear from the account which Nebuchadnezzar II. has left us of his famous fortifications of Babylon,[29] which are referred to by Greek writers as "the Median Wall" and "the Fortification of Semiramis." The outermost ring of Nebuchadnezzar's triple line of defence consisted of an earthen rampart and a ditch, which he tells us extended from the bank of the Tigris above Opis to a point on the Euphrates within the city of Sippar, proving that Opis is to be sought upon the former river. His second line of defence was a similar ditch and rampart which stretched from the causeway on the bank of the Euphrates up to the city of Kish. It was assumed that this rampart also extended to the Tigris, although this is not stated in the text, and, since the ideogram for Opis is once rendered as Kesh in a bilingual incantation,[30] it seemed probable that Kish and Opis were twin cities, both situated on the Tigris at no great distance from each other. This view appeared to find corroboration in the close association of the two places during the wars of Eannatum, and in the fact that at the time of Enbi-Ishtar they seem to have formed a single state. But it has recently been shown that Kish lay upon the Euphrates,[31] and we may thus accept its former identification with the mound of El-Ohêmir where bricks were found by Ker Porter recording the building of E-meteursagga, the temple of Zamama, the patron deity of Kish.[32] Whether Opis is to be identified with the extensive mounds of Tell Manjûr, situated on the right bank of the Tigris in the great bend made by the river between Samarra and Baghdad, or whether, as appears more probable, it is to be sought further down stream in the neighbourhood of Seleucia, are questions which future excavation may decide.[33]