Ngazargamu
Ngazargamu, located in today's Yobe State along the Yobe River, had an estimated population of 60 thousand in 1600, making it tied for the largest city in what is now Nigeria.1 It and was the capital of the state of Bornu.2 It's relative status was overtaken by Katsina in subsequent centuries and it was destroyed in 1808.2 Its residents were Kanuri speaking and were Moslems. It is also called Kazargamu.
Before the 20th century, the city name was rendered in Ajami script, perhaps like this3: كزرغمو
Footnotes
(1) In Chandler's table, Cities of Africa, 1600, in the section, South Moslems, Ngazargamu (Kazargamu) is followed by Zaria and Katsina; in the Guinea section Oyo is first. All four have the same estimated population, 60 thousand (Tertius Chandler, Four Thousand Years of Urban Growth, 2nd ed., 1987, The Edwin Mellen Press).
(2)ibid., pg. 286.
(3) Possible errors. Vowels are mostly omitted here. Normally Arabic syllables that are consonant-vowel end in a long vowel, and long vowels are represented orthographically, and long u is normally one of them, so I represented the final vowel with a waw. But it is not clear how the Ajami script (adaption of Arabic script to non-Arabic) would handle the other vowels nor the 'r', which is a vowel modifier, at least in the English pronunciation of the city's name. Nor is it clear if the 'n' of the 'ng' would be represented; I omitted it. Further the choice of kaf rather than qaf or gayn is speculative.