It could be about a character who is trapped in a tower. Maybe they are facing various challenges while being stuck there, like trying to find a way out, encountering strange creatures or having to solve puzzles within the tower.
Typically, in a light novel with this kind of premise, the main character is likely to be an ordinary person thrust into this extraordinary situation of being stuck in the tower. There may be other characters such as friends or foes that the main character meets along the way. For example, there could be a group of other trapped individuals who form an alliance with the main character, or there could be an antagonist who is trying to keep the main character in the tower for some ulterior motive.