The Thongor series is Lin Carter's pastiche of John Carter of Mars mixed with Conan. The first Thongor novel was actually the first Carter book ever published, and I gather led to him getting the job editing (and, well, writing) the Ballantine Conan series. City of Magicians is #4 in the series. The only other one I've read is #3 - I haven't managed to get hold of #1 yet, though #6 is on my To-Be-Read stack - but given the episodic nature of the series, that hasn't been a problem so far.
At this point in the series, Thongor is king of a bunch of cities. His primary opponent is the city of Zaar, city of wizards, who (in what I must admit is a nice touch) gained their dreadful power by being the only human power to betray humanity and ally with the evil Serpent-Men back in the great war. Thongor's philosopher(s) have figured out how to build an electric ray-gun to equip their flying boats with, and so he goes off with part of the Army to harvest the jewels needed to build the ray-guns. While this is going on, he's captured by agents of Zaar, and the rest pretty much unfolds as you'd expect it to.
I'll be blunt: I did not like the previous Thongor book I read, and gave it 2/5. This one's better. It's not great fantasy or anything - certainly nothing that would justify giving him license to write new Conan stories - but it's decent, workmanlike. My main complaint is the ending, which includes literal divine intervention, as well as a bunch of really contrived coincidences. I'll give it 3/5.
Incidentally, one thing I noticed while reading this: I can really see the influence this series had on Geoffrey McKinney's Carcosa setting. McKinney did it way, way better though. Which is kind of odd; usually one thinks of a great master inspiring lesser disciples, rather then the other way round.