Kouka Koukyuu Monogatari Nakamaku Itoshiki Tasogare

Shougyoku Kan was renowned as the strongest female soldier. But, for some inexplicable reason, she, still single after turning 30, is chosen to become empress and enters the "Crimson Mist Palace," a harem swirling with female jealousy and desire—!? (Source: Kadokawa, translated)