A lot of running around trying to avoid bad guys, and I never fully got attached to the characters, to be honest, so I didn't get absorbed in the book or the romance. Exquisitely, flawlessly beautiful heroines with a tragic past get less effective after a while, as do rich, dashing aristocrat heroes, when that is all we know about them.
The reader is automatically expected to side with the royal family against the rebels, who were generic "bad guys." We don't know much about this royal family from this volume, so perhaps I would have agreed with the rebels (aside from their use of violence, of course, but they were hardly the only violent ones in the book). This left me feeling ambiguous about the action, to be honest, and diminished my connection with the main characters. I might feel differently if I had read the whole series previous to this, I admit, but this book by itself left me feeling uncomfortable with this aspect.
A few interesting things - the age difference, with hero younger, and the heroine being a (presumably) wealthy celebrity in her own right was unusual, as was some of the catacomb descriptions.