What was the real-world inspiration for the Kingsguard?

Question

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.}

Answer 1

There have been (elite) guard units throughout history, such as the cohortes praetoriae and the numerus Batavorum during the Roman Empire, the Pushtigban and the Gond-i Shahanshah of the Sassanid Empire, etc.

However, due to the celibacy, the first organisation that comes to mind is the Pontifical Swiss Guard:

Recruits to the guards must be unmarried Swiss Catholic males between 19 and 30 years of age who have completed basic training with the Swiss Armed Forces.

A more recent source of inspiration may have been the Greek Evzones or Evzonoi:

Prospective recruits had to be of good health and character, over 17 and under 40 years of age, and preferably unmarried.

Some members of the Kingsguard are of noble descent (most notably Jaime Lannister), but they don't need to be, unlike, for example, the Vatican's Noble Guard (1801-1970).

However, I don't think that the oath is "for life" in these organisations; George R. R. Martin likes taking historical examples to an extreme.

Answer 2

Possibly some inspiration came from the Order of the Garter.

During a general interview regarding question on the Kingsguard he mentioned the Knights of the Garter.

Institutions like the Kingsguard change over time. The original Knights of the Garter were warriors all, the strongest, bravest, deadliest men of their time, with an average age under thirty. The present Knights of the Garter are octogenarians, and their parades are processions of wheelchairs and walkers.

The most direct comparison I can see is that the Order was formed to help Edward III pursue his claim on the French throne. The Kingsguard helped Aegon I keep his throne...