The Morgaine books, despite their high fantasy trappings — elves, magical sword, medieval Northern Europe-inspired settings — are really intimate studies of the main characters' relationship. (They are also, technically speaking, science fiction — e.g., the elves are actually the alien qhal — so that, somehow, these books are part of Cherryh’s Alliance-Union universe.) Morgaine is Vanye’s liyo, meaning that he has sworn an oath to her that requires him to obey her commands but also implicitly requires her to take responsibility for his well-being (up to a point, at least). But though the oath is deeply serious to Vanye, and Morgaine does him the courtesy of treating it as such as well even though it’s not a thing of her culture, it has been removed from a context where it makes sense, and as such is constantly under strain. Vanye’s problem is that he has sworn an oath to someone whose motivation is completely unfamiliar to him. Morgaine doesn’t care about land or honor or any of the things that his culture prioritizes: she only cares about her quest, and all her actions are judged solely according to whether or not they help to advance it. This means that Vanye’s oath regularly brings him into conflict with what he feels is the right thing to do, something that would not be a problem if he had sworn it to someone from his own culture who would have the same ideas about right and wrong that he does. Given that Morgaine is half qhal, Vanye tends to think of these conflicts as requiring him to deny his humanity, which raises their stakes considerably. However, there’s another force at play here, which is their increasing affection for each other: it’s a little cliched, but considering that Morgaine is the only remnant of Vanye's old life, and Morgaine hasn’t had so much as a friend in who knows how long, it’s not entirely surprising. The push and pull between these different aspects of their relationship gives the books much of their interest and tension.
Otherwise, “Exile’s Gate” is much the same as the first three, only longer: Morgaine and Vanye show up on a new world and the locals try to stop them from destroying their Gate. Even the world is fairly similar to the previous worlds: this is necessary, I think, to put the maximum pressure on Vanye’s oath — he wouldn’t feel that he was betraying his humanity by siding with Morgaine against the local humans if they weren’t part of a feudal society similar to the one he grew up in — but it nonetheless makes the books less distinct than they might be, and sacrifices one way in which Cherryh could juice their interest a bit. There is a twist here, namely that there are more qhal than usual and they are using the Gates to prolong their lives by transferring their consciousnesses into new bodies. But otherwise it’s the usual formula: luckily, it’s a pretty good formula. Even with the longer running time, Cherryh effortlessly maintains suspense, and there are several neat twists along the seemingly straightforward route from A to B depicted in the map at the front of the book. Plus, there are the characters: not just Morgaine and Vanye, but also the locals, who are struggling not just against Morgaine and Vanye but, often enough, each other. Only the final boss villain is lacking: we don’t get enough of a sense of him during the book to give the final confrontation the heft it needs. Still, this is mostly the story of Morgaine and Vanye, and is quite successful as such. While I recognize that there was no need for another book after this one — at best, it could only be a rewrite of this — I wouldn’t have minded reading it anyway.