John Flanagan 
Ranger’s Apprentice The Royal Ranger 5: Escape from Falaise [EPUB ebook] 

Will and Maddie are imprisoned at Chateau des Falaises. Baron Lassigny is scheming to recruit them as spies for hire, but Maddie raises the stakes when she challenges Lassigny’s garrison commander to a duel on horseback. Will is horrified — especially when he realises the treacherous baron won’t fight fair. If Maddie can survive the duel, they still need to escape the heavily guarded chateau and rescue the kidnapped prince they were sent to retrieve. It will take all their ingenuity, courage and determination to triumph this time — and to solve the mystery of who betrayed them.