The Princelings of the North

From the Publisher:

Dylan and Dougall are princelings at Castle Haunn, a remote place to the far northwest of an island off the coast of Scotland. So when they discover a prince locked in a tower, their thoughts turn to rescue and returning him to his rightful place in a castle hundreds of miles away. But nothing is ever that easy, and what starts as a simple mission turns into a nightmare that rocks the foundations of the Realms.