I can't remember the name of this novel. Non-traditional princess is sent across space by her father the king to marry the crown prince as a diplomatic move. There are priests and a type of angelic presences. The princess set up sword-fighting lessons for the ladies of the court and scandalized everyone. The prince gets a disease that causes him to be exiled to a walled city where the diseased are sent to die. There are factions among the exiled. Princess eventually goes to the city. I think the prince was hiding his identity. Eventually, there was a battle against the king's soldiers.
This was probably published 3 to 5 years ago. I listened to the audio-book. It seemed like it may have been the first of a series but I'm not positive. My library had it classified as science fiction but it had a lot of fantasy elements.