George RR Martin's fantasy series A Song of Ice and Fire takes a lot of inspiration from real-world history: Hadrian's Wall becomes the Wall in the North, Yorkists and Lancastrians become Starks and Lannisters, and the French Foreign Legion (maybe?) becomes the Night's Watch. What about that other organisation of men who take oaths for life and swear to celibacy, the Kingsguard?
Is the Kingsguard based on or inspired by any real-world organisation?
Answers based on quotes from GRRM, compelling similarities, or any other good evidence will be welcomed.