Maoujou de Oyasumi

The Demon Lord kidnaps Princess Syalis from the human kingdom and locks her inside a castle full of monsters, expecting her to spend her captivity terrified until the heroes come to rescue her. The princess cares very little about the war or who eventually defeats whom. Her real problem is that the prison bed is terrible and she cannot get a decent night's sleep. Syalis begins roaming the castle in search of materials for a better pillow, softer blankets, and quieter places to rest, happily cutting monster fur, dismantling equipment, or risking her life if it might improve her sleeping conditions. Before long, her guards are the ones living in fear, because a prisoner who does not fear death and sees every creature as potential bedding may be more dangerous than the army waiting outside.