Cult of Nabu at Calah
“As for Bel, whose place Merodach usurped in the Babylonian pantheon, he was also recognized in Assyria, and Tiglath-pileser I built him a temple in his city of Asshur. Tiglath prefixes the adjective ‘old’ to the god’s name to show that he means Bel, not Bel-Merodach.
Sargon, too, who had antiquarian tastes, also reverts to Bel, to whom he alludes as the ‘Great Mountain,’ the name of the god following immediately after that of Asshur. Bel is also invoked in connexion with Anu as a granter of victory.
His consort Belit, although occasionally she is coupled with him, more usually figures as the wife of Asshur, and almost as commonly as a variant of Ishtar. In a temple in the city of Asshur, Tiglath-pileser I made presents to Belit consisting of the images of the gods vanquished by him in his various campaigns.
Assur-bani-pal, too, regarded Belit as the wife of Asshur, and himself as their son, alluding to Belit as ‘Mother of the Great Gods,’ a circumstance which would go to show that, like most of the Assyrian kings, his egoism rather overshadowed his sense of humour.
In Assur-bani-pal’s pantheon Belit is placed close by her consort Asshur. But there seems to have been a good deal of confusion between Belit and Ishtar because of the general meaning of the word Belit.
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Stephen Thompson – Asshur, Assyrian God, Marble Relief, British Museum, 1872
The Catalogue of a Series of Photographs from the Collections of The British Museum (Photographed by S. Thompson), Part III, W.A. Mansell & Co, London, 1872, p. 30.
In the Nimrud Gallery of the Museum, Eastern side, #355. B.C. 884.
Marble slab. Eagle-headed winged deity Asshur (the chief of all the Gods), holding cone and basket, (supposed to represent the receptacle in which the divine gifts are stored) and standard inscriptions.
https://www.flickr.com/photos/photohistorytimeline/10171487505/
As in Babylonia so in Assyria, Nabu and Merodach were paired together, often as Bel and Nabu. Especially were they invoked when the affairs of Babylonia were being dealt with. In the seventh century b.c. we find the cult of Nabu in high popularity in Assyria, and indeed Ramman-Nirari III appears to have made an attempt to advance Nabu considerably.

George Rawlinson – Source: Seven Great Monarchies Of The Ancient Eastern World, Vol 1. (1875)
The Chaldean god Nebo, from a statue in the British Museum.
http://www.totallyfreeimages.com/56/Nebo.
He erected a temple to the god at Calah, and granted him many resounding titles. But even so, it does not seem that Ramman-Nirari intended to exalt Nabu at the expense of Asshur. Indeed it would have been impossible for him to have done so if he had desired to.
Asshur was as much the national god of the Assyrian people as Osiris was of the Egyptians. Nabu was the patron of wisdom, and protector of the arts; he guided the stylus of the scribe; and in these attributes he is very close to the Egyptian Thoth, and almost identical with another Babylonian god, Nusku ...
Sargon calls Nabu ‘the Seer who guides the gods,’ and it would seem from some notices of him that he was also regarded as a leader of heavenly or spiritual forces.
Those kings who were fond of erudition paid great devotion to Nabu, and many of the tablets in their literary collections close with thanksgiving to him for having opened their ears to receive wisdom.”
Lewis Spence, Myths and Legends of Babylonia and Assyria, 1917, pp. 227-9.