Often labelled the world's smallest army and founded under Pope Julius II, the Swiss Guards have celebrated 515 years guarding the leader of the Roman Catholic Church.

It was Pope Julius II that founded the Swiss Guards in 1506. Often dubbed as "the world's smallest army"

Founded in 1506, the Swiss guards - a separate entity to the Swiss Armed Forces - are hired by the Roman Catholic Church, under the leadership of the Pope, Vatican News reports.

Member of the all-male unit swear loyalty to the Pope in a ceremony at the Belvedere Court in the Vatican.

This ceremony takes place every year on May 6 - to mark the anniversary of the Sack of Rome in 1527.

That year over 150 Swiss Guards were killed defending Pope Clement VII - who escaped to neighboring Castel Sant'Angelo, escorted by 22 remaining guards.

WHY SWISS?

It is said that during the Middle Ages, Switzerland gained a reputation for having the most reliable mercenaries in Europe.         

Then Pope, Julius II, who had long admired the Swiss soldiers who defended the King of France.

He requested 200 Swiss mercenaries of his own to use as bodyguards and to defend his palaces. After months of marching, 150 swiss guards arrived in Rome on Jan. 21, 1506 to serve the Pope.

SWISS GUARDS TODAY

In 2015 the contingent was made bigger, from 110 guards to 135 to meet requests.

Each recruit is required to complete, military training in Switzerland.

This is followed by a five-week induction training course in Rome, and finally a period dedicated to training to use the primary weapons that the mercenaries used in the 16th century.

After this, the guards are sworn in and their families are invited to a private audience with the Pope.

Along with the Pontifical Gendarmerie, the Swiss Guards are responsible for ensuring the safety of the Pope both within and outside the Vatican.

They also accompany Pope Francis as he moves around among the people.

"Today the traditional Renaissance uniform is worn by modern, well-trained, young Swiss," says the official website of the Swiss Guard.

It says the modern guard is connected to his 16th century predecessors by his "firm conviction that he serves Christ's Church and his Vicar on Earth, the successor of Saint Peter, that he will sacrifice his own life, if necessary, to protect the Pope.

Recruits to the guards must be Catholic, single males with Swiss citizenship who have completed basic training with the Swiss Armed Forces and can obtain certificates of good conduct, writes David Alvarez author of The Pope's Soldiers.

Recruits must have a professional degree or high school diploma and must be between 19 and 30 years of age and at least 174 cm (5 ft 8.5 in) tall and they must  be able to speak Italian.