A Song of Stone by Iain Banks

A European nation not unlike Bosnia: armed forces roam the lawless land where dark columns of smoke rise up from the surrounding farms and houses. The war is ending, perhaps ended. But for the castle and its occupants, a young lord and lady, the trouble is just beginning.
Fearing an invasion of soldiers, the amorous couple takes to the road with the other refugees, disguised in rags. But the brutal female lieutenant of an outlaw band of guerrillas has other ideas. Just hours into their escape, the fleeing aristocrats are delivered back to the castle, where, now prisoners in their own home, they become pawns in the lieutenant's dangerous game of desire, deceit, and death.

Used condition.