The Crown Tower by Michael J. Sullivan (The Riyria Chronicles #1)

Two men who hate each other. One impossible mission. A legend in the making.

Hadrian Blackwater, a warrior with nothing to fight for is paired with Royce Melborn, a thieving assassin with nothing to lose. Together they must steal a treasure that no one can reach. The Crown Tower is the impregnable remains of the grandest fortress ever built and home to the realm's most prized possessions. But it isn't gold or jewels that the old wizard is after, and if he can just keep them from killing each other, just might do it.

The Riyria Revelations and The Riyria Chronicles are two separate, but related series, and you can start reading with either Theft of Swords (publication order) or The Crown Tower (chronological order).