The English archaeologist A.H. Layard excavated Nimrud in 1840 and found there the palace of Ashurnasirpal II. Most reliefs sculptures and reliefs are now at the British Museum in London. They describe, in the year 879 BCE, that Ashurnasirpal II organized a great festival to celebrate the end of the construction of his new capital, Kalhu, and its palace. On this occasion it would have invited 69,000 people, including 16,000 new citizens of Kalhu and 5,000 dignitaries of its territory or foreign countries.
In the Neo-Assyrian texts, this town is sometimes spelled Kalah, Kalach, Calach. The excavations showed that it existed during the previous millennium.
Kalhu is spelled Kawalhum in the archives of Mari. II 39 refers to this city that was given to Asqur-Addu king of Karana, who also controlled Qattara. This set of towns was then under the threat of Mut-Asqur, son of Isme-Dagan, installed in Razama at the head of an army of 2000 Babylonian soldiers reinforced by 2000 soldiers of Ekallatum and Assur.
Kalassa is another spelling, especially towards the end of the reign of Zimri-Lim: A.1121 + A2731 refers to a prophet of Addu lord of Kallassu, probably the neo-Assyrian Nabu, whom had a temple in Nimrud.
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My notes of veracity :
During the the first millennium BC, Nimrud was Kalhu: 5/5
During the second millennium BC, Nimrud was Kawalhum or Kalassa: 3/5