1 Chronicles 11:7

And David dwelt in the castle; therefore they called it the city of David.

David lived in the stronghold; therefore they called it the city of David.

And David dwelt in the stronghold; therefore they called it the city of David.

And David dwelleth in the fortress, therefore they have called it, `City of David;'

So David took up residence in the fortress; that is why it was called the City of David.