Oathbreaker. The Horn, #1

Amal, Lady Horn, and her guards are charged with preserving one of the great secrets of Larossa: an abandoned Fortress. The chief of the Oathbreakers, Amal is one of only a handful of people aware of the true dangers the abandoned Fortress of Salonen presents. The Cince Empire wants its secrets, and will do anything to get    someone inside. Now the Horn Family must decide whether to wake the sleeping Fortress so it can defend itself against the Cince.or kill it forever. Dalyan doesn't know why he was sent to find the abandoned Fortress, what makes it worth the time his masters invested in training him.
When the Horn arrest him for trespassing on the glacier beneath it, he goes with them willingly enough. After all, he didn't return to his masters on schedule, and now they're trying to kill him. But more than that, he feels drawn to the abandoned Fortress, as if he belongs there.