A great fanfic about Julius Kingsley in Code Geass should have a strong characterization. It should capture his personality as it was in the series, with all his mystery and complexity.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Code Geass fanfics, including those about Julius Kingsley. Just use the search function and type in 'Julius Kingsley' to get relevant results.
I think 'Take a Girl Like You' is among his best. It tells the story of a young woman, Jenny Bunn, and her experiences in love and society. The novel shows Amis's ability to create complex characters and engaging storylines. It also gives an interesting view of the social norms and expectations of the time.
Julius Caesar's story is based on actual history. While some details might have been embellished or simplified for storytelling purposes, the main events and his existence are factual.