Shalmaneser thus, it would appear, gained control of northern Mesopotamia and consequently of the Babylonian caravan route to Haran. As a result Hittite prestige must have suffered decline in Babylon. For a generation the Hittites had had the Babylonian merchants at their mercy, and apparently compelled them to pay heavy duties. Winckler has found among the Boghaz Köi tablets several letters from the king of Babylon, who made complaints regarding robberies committed by Amoritic bandits, and requested that they should be punished and kept in control. Such a communication is a clear indication that he was entitled, in lieu of payment, to have an existing agreement fulfilled. Shalmaneser found that Asshur, the ancient capital, was unsuitable for the administration of his extended empire, so he built a great city at Kalkhi (Nimrud), the Biblical Calah, which was strategically situated amidst fertile meadows on the angle of land formed by the Tigris and the Upper Zab. Thither to a new palace he transferred his brilliant Court. He was succeeded by his son, Tukulti-Ninip I, who was the most powerful of the Assyrian monarchs of the Old Empire. He made great conquests in the north and east, extended and strengthened Assyrian influence in Mesopotamia, and penetrated into Hittite territory, bringing into subjection no fewer than forty kings, whom he compelled to pay annual tribute. It was inevitable that he should be drawn into conflict with the Babylonian king, who was plotting with the Hittites against him. One of the tablet letters found by Winckler at Boghaz Köi is of special interest in this connection. Hattusil advises the young monarch of Babylonia to "go and plunder the land of the foe". Apparently he sought to be freed from the harassing attention of the Assyrian conqueror by prevailing on his Babylonian royal friend to act as a "cat's paw". It is uncertain whether or not Kashtiliash II of Babylonia invaded Assyria with purpose to cripple his rival. At any rate war broke out between the two countries, and Tukulti-Ninip proved irresistible in battle. He marched into Babylonia, and not only defeated Kashtiliash, but captured him and carried him off to Asshur, where he was presented in chains to the god Ashur. The city of Babylon was captured, its wall was demolished, and many of its inhabitants were put to the sword. Tukulti-Ninip was evidently waging a war of conquest, for he pillaged E-sagila, "the temple of the high head", and removed the golden statue of the god Merodach to Assyria, where it remained for about sixteen years. He subdued the whole of Babylonia as far south as the Persian Gulf, and ruled it through viceroys.