C. L. Moore and Henry Kuttner are two of the most revered names from SF's Golden Age, and are especially famous as the genre's most high-profile husband-and-wife collaborators. Both published their first stories in the legendary pulpzine Weird Tales, and corresponded with H. P. Lovecraft. Moore was one of the very first women to write prominently in the field, though her use of initials, and even her husband's byline outright on some occasions, testifies to the very male-dominated publishing culture that was still decades away from gender parity. Kuttner is said to have been a strong influence on Philip K. Dick and Roger Zelazny. Together they wrote a number of stories under their shared byline, as well as such pseudonyms as Lewis Padgett. One of these — the 1943 short story "Mimsy Were the Borogoves" — served as the basis for the 2007 children's movie The Last Mimzy. Kuttner died in 1958, Moore in 1987.
Having established these authors' historical bona fides, here comes the rude middle finger of reality: Most SF from this period, though it has immense historical value, doesn't hold up today for a picosecond. The wartime adventure Earth's Last Citadel is no exception. It's all about a quartet of people — two Allies, and two agents for the Axis — who stumble upon a mysterious craft in the North African desert, and are catapulted into the far far future, where they end up in conflict on a dying Earth against the mysterious alien "gods" who control a specially-bred species of pet humans from their towering citadel. While I could see how it all may have been quite thrilling six decades ago, this castaways-in-time saga is, by any modern measure, fairly dull and bland reading, even at under 150 pages. There is no depth to any of the stock characters, and the story relies too heavily on dazzling imagery for its sense of wonder, an approach that may have been more successful had the prose used to convey said imagery been substantially less purple. The usual clichés put in an appearance; the first human encountered in this distant future is a beautiful girl, who promptly hooks up with our stalwart male protagonist.
There's nothing here that's dreadful. It's just too dated and formulaic to have anything other than novelty value today. If you're looking for examples of early SF from a scholarly point of view, in order to understand the evolution of the genre's storytelling techniques and tropes, then digging up old relics like Earth's Last Citadel will be worth your while. Fans looking for something entertaining to read are advised that there are over 60 years' worth of material since this came out that they're much more likely to find relatable.