The Engineered Throne. Unbreakable Soldiers, #1

Lord Vellem is desperate to escape. His father hides from his problems in alcohol, his mother hides from her misery in drugs, and his brother fled to the relative safety of the royal palace. Vellem found solace in the Royal Army Corp of Engineers, but true escape from a life of violence remains out of reach. Then his brother provides him with an unexpected chance: marriage to a prince of their country's oldest enemy to help bring about peace once and for all.
The marriage promises Vellem not just a fresh start in a new land, but a challenge to his famed skills in building roads and bridges. But before he can settle into his new life, tragedy strikes, and Vellem isn't certain he's up to rebuilding the ruin left in its wake.