The great mound, the Kasr or castle, forms the centre of the city. It is the great castle of Nebuchadnezzar that he built for a palace, completing the work of his father, Nabopolassar. The modern name Kasr thus expresses the purpose for which it was built. By Greek historians it was called the Acropolis, by Romans the Arx. In area it is three or four times as large as Babil, but it is not so high, and when observed from that hill the greater part is hidden by palms. This Acropolis, built on what is called the _Irsit Babylon_ (_Steinplatten_ inscription, col. 7 l. 40), the piazza or town square of Babylon, is actually the original Babylon, the _Bab-Ilani_, the Gate of the Gods. It commanded the approach to the greatest and most renowned sanctuary of Babylonia, the temple of Marduk called Esagila. This lies somewhat farther to the south, buried 20 metres deep under the great hill, the third of the three great mounds of Babylon, Amran Ibn Ali, a name acquired from the sanctuary which is upon it, the tomb of Amran the son of Ali. It is 25 metres high, the highest of all the mounds, and owes this to the fact that after all the other sites were abandoned it was occupied for habitation right up to the Middle Ages, under Arab rule. Close by to the north lies the rectangular ruin of the tower of Babylon, E-temen-an-ki, on a small plain called Sachn, that represents its sacred precincts. Due east of the Kasr a smaller but unmistakably higher mound rises from the plain, called from its red colour Homera. It conceals no buildings, but from top to bottom it consists of brick fragments. We shall return to it later. Close by, almost due north and south, extends the low ridge of ruins of the inner city wall that encircled the inner portion of the city in a line not yet fully traced. Between Homera and Amran, as well as to the south of the latter, and between the Kasr and Babil, we see the plain broken by a number of low mounds distributed in groups. Here clustered the dwellings of the citizens of Babylon, and the recollection of them has so far survived to the present day that one of these groups south-east of the Kasr is called by the Arabs Merkes, the city or centre of the dwellings. It is here that the dwellings and streets of the city of the time of the Persian kings, and as far back as that of the earliest Babylonian kings, have survived in the mass of ruins. Externally these remains present the appearance of mountainous country in miniature; heights, summits, ravines, and tablelands are all here. At Merkes there is a sharp hill visible from a distance, due to an excavation previous to our expedition when the rubbish dug out was collected there. There are also public buildings buried in the ruins. Thus between Homera and Merkes there is a Greek temple, on Merkes itself is a temple, and there are two in the so-called Ishin aswad, the district south-east of Amran.