If you don’t know The Prisoner of Zenda, then I doubt this will be of much interest — and I think you’d also have to be interested in m/m romance to really appreciate it, as well. With both those things in mind, it’s a fun romp, turning aspects of The Prisoner of Zenda on their head and making a little more of two background characters, and particularly the flamboyant and unrepentant Rupert of Hentzau. Detchard is a total cipher in the original book, really, so he makes good ground for Charles to play with in rewriting the story.
Ultimately, is it an amazing work of literature? No. Does it do anything particularly new or interesting in rewriting The Prisoner of Zenda? Also no (though I enjoyed that Charles didn’t try too hard, Black Michael is still as black as he’s painted). It doesn’t depart too far from the original, though I enjoyed the friendship between Detchard and Antoinette, and Rupert’s actual loyalties, as additions to the original plot.
It may not be very startling, but it’s solid fun, and I do enjoy the bantering, slightly adversarial relationship between Detchard and Rupert. Charles doesn’t lay it on too thick or try to change the essentials of the characters, and it produces a good romp.