I first encountered this book via a very loose adaptation as an episode of the Saturday morning cartoon _Tarzan, Lord of the Jungle_, namely, "Tarzan and the Knights of Nimmr", which took the idea of a lost civilization founded by Crusaders, then oversimplified it while coupling it with a different plot (that is, changing the specific characters involved and the manner in which an outsider discovers the lost valley).
Burroughs often played with the idea of civilizations in which two cities, locked into patterns of eternal warfare, remained cut off from the rest of the world, but they're all different. The two cities occupying the isolated African valley where this story takes place were settled centuries ago by a few shiploads of lost crusaders, who had picked up some women on their travels, soldiers being soldiers even on crusade. The Crusaders split into rival factions when they discovered the valley, one faction (insisting that they had achieved the Holy Grail and thus the Crusade) founding the City of the Sepulcher, the other (denying it) founding the city of Nimmr, guarding the valley's only exit and preventing the rival faction from going home.
While officially the issues haven't changed, in fact the two cities continue to fight because that's what they've always done. (That seems realistic enough, considering Ireland, the Middle East...) The valley is the only home they've ever known, and if either ever really 'won' the war, they know that proceeding to either the Holy Land or to England would be fraught with problems. They've made accommodations with each other for survival, some of which are very far-sighted. For example, periodically a truce is declared and a great tournament held between the two cities, in which the grand prize provided by the losing city to the winner includes 5 highborn maidens. The winning city's ruler arranges honourable marriages for them - thus ensuring that the valley's population doesn't become dangerously inbred.
The specific details of how an outsider stumbles across the lost valley are somewhat less happily handled, although once he's in, the story smoothes out. James Blake is an American explorer with a bad guy for a partner, and their 'native' support team is handled in a stereotypical manner - although the bad guy is the racist, so one might be able to cut Burroughs some slack for the sake of the Nimmr/City of the Sepulcher bulk of the story. (On the plus side, the local villagers are nobody's fools.) Tarzan becomes aware of the party passing through his territory, and when Blake is separated from his crew, Tarzan takes a hand. Tarzan's really a supporting character for most of the story - Blake is the protagonist.