世界の果ての物語

Kou is a girl who believes in her sword and she sets off to the capital city. However, she is caught by the enemy country army on the way, and is imprisoned in the Gallsberde fortress. She was offered to gather bail in 30 days time or die. Unnatural events start to happen around her. Now she needs to discover the truth about those events and make it in 30 days time.

[From vndbreview]

In a grey world, our girl is a prisoner in an enemy castle. Her next cellmate found a way for her to move around the castle, but it is still 30 days before she gets ransomed. She can either explore the castle, meeting people or do some sabotage work instead. Whatever she does, hope she has fun before the 30 days are up.