Hammer & Powder by Megan Derr

The Kingdom of Rinaha is all that stands between the violent, greedy Boorna and the rest of the continent, and they do so by way of their immense and heavily guarded Wall of Gamala, an enormous undertaking that runs the length of the southern end of the continent, standing strong for more than two hundred years.

Maintaining the Wall and the forces that guard it is an expensive undertaking, one that other countries are reluctant to contribute significantly to, despite the fact they benefit greatly from its existence. In an effort to change that, the king of Rinaha arranges a marriage between the powerful Takahara family and a royal relation of the neighboring kingdom of Aaran.
Hideki, better known as Warhammer, is playing escort for his brother Saburo, the chosen candidate being sent off to marry the nephew of the king of Aaran and bridge political divides once and for all. Shortly after arrival, though, Saburo runs away, with only a letter of apology left to explain what he’s done. Now it is Hideki, ill-suited to political games, who must keep anger and wounded pride from erupting into war.

Meanwhile, back on the Wall, his twin brother Daisuke discovers the enemy is using children to do what their soldiers cannot, and in rescuing them, finds himself in exactly the sort of situation he prefers to avoid. On top of that, his brother Jiro, General of the Wall, expects him to entertain foreign guests, despite the fact everyone knows not to trust delicate situations to the man known as Gunpowder.