The King's Mane - I'm an Old Man Inside, But I'll Do My Best -

Long ago, Isaku Takara, who had gone to war as a member of the Air Self-Defense Force, died in a fire. Well... he was already aging, and given his age, he had simply weakened and passed away, so there was nothing to regret. Fortunately, he had been able to see his grandson's face. After that, he thought he would peacefully watch over his wife who had passed before him and his daughter who had married and gone away, from heaven. That's what he thought. But somehow, Isaku was reborn into a world of swords and magic. Moreover, he was a child of a certain country's king, confined in a tower with only one window—a mysterious situation. ...How did this happen? At the subtle age of 15, Isaku remembers his past life and rises up to break through his current situation!! His mind is an old man's, but he'll make it work through sheer willpower!! But why am I a girl? I would be grateful if you would watch over this story of a somewhat mischievous grandfather surviving in another world!! Since I post at my own very leisurely pace, please enjoy it as a way to pass the time!!