While reading I kept thinking that the Musketeers have grown up. The plot is more complex and actual historical events play a much larger role. Things are also less black and white, at least mostly. Mordaunt, the main villain is still pretty much plain evil. True his back-story makes you question what would have happened if Lord Winter had treated him differently but none of his actions during the book evoke any sympathy.
Despite those differences the important things are still the same as the first book. It's exiting, even though I knew they would not be able to save Charles because...well history, I was still glued to the pages and felt shocked when he got executed. The friendship between the four characters is also just as powerful as in the first book. Perhaps even more because they hadn't seen each other in ages but still fall back in the old ways almost immediately.