I read this novel years ago, so I only remember some scenes, and even that is uncertain. It is a medieval fantasy novel. The FL is a famous soldier from the battlefield heading to the capital because the queen is holding a banquet, and she is the representative of some noble. On the way to a village, she met the ML, and they somehow ended up being attacked in the inn. There the ML realizes that FL isn't that naive and can fight pretty well. During the banquet, a male character somehow pours wine on her (I don't remember why), then she goes away to change and comes back wearing her soldier uniform, and everyone is surprised. The queen also wants the nobles to go into another realm (I don't remember what, but they went there through a magic gate) with their soldiers. Of course, the FL goes too, and it seems like she is already familiar with the place. There is nothing but sand or ash, you don't have to eat, drink or sleep, but they eat salt. There are also some monsters (maybe sceletons?), and their objective is to get back someone's 'heart', which turns out to be a ring. Naturally, the FL gets it back, even though she went alone. The ML also went with his soldiers, and with the help of the FL's information, he got a lot of gold and treasures. At some point, they also form an alliance and plan to marry. I'm sorry if it's confusing... I would be really thankful if you knew the title!