Its massive walls, topped by stone eagles and statues of saints, dwarf the crowds of tourists queuing to see the treasures inside its museums. The Vatican may look like a medieval fortress, but the apparent impregnability of its bastions, buttresses and revetments is illusory.
Not even the Swiss Guards in their flamboyant harlequin uniforms managed to prevent the worst security breach in the Holy See's recent history - the theft of hundreds of confidential documents, some of them stolen from the desk of Pope Benedict himself.
Smuggled out of the secretive city state, they were handed to an Italian journalist, Gianluigi Nuzzi, who dropped a metaphorical bomb on the Holy See by publishing them as a book: His Holiness - The Secret Papers of Benedict XVI.
The sensitive documents paint a startling picture of the Vatican in disarray, with the 85-year-old Pope apparently too wrapped up in his theological musings to prevent toxic feuds and vendettas between rival factions in the Curia, the opaque, archaic Vatican bureaucracy.
One person has been arrested so far on suspicion of taking the letters – Paolo Gabriele, the Pope's 46-year-old butler, who is now languishing in a 12ft by 12ft cell (which Vatican authorities prefer to call a "secure room") deep within the walled city state.
Nicknamed within the Vatican "Paoletto", or little Paul, he is one of a select handful of people who had daily access to the Pope in his private apartments, and was a trusted member of the pontiff's household.
His arrest, which followed the discovery of a pile of stolen documents in his Vatican apartment, was "a heavy blow", his wife said.
"My husband has always wanted the best for the Church. I am convinced that he would never have done something that could damage the Holy Father," Manuela Citti, a housewife and mother of three, told Italian journalists.
"For us in the family, but also for the many friends who have known Paolo for years, it was a heavy blow – unexpected and inexplicable. But we have confidence in the (Vatican) magistracy and we believe everything will be clarified."
Mr Gabriele, who has worked for Benedict since 2006 and previously served John Paul II, is being held within the headquarters of the Vatican Gendarmerie, the city state's 130-strong police force.
The cells are some of the least used in the world – they occasionally hold pickpockets caught in St Peter's Square but the last time all three were full was in 1971, when four employees of the Vatican's telephone exchange were accused of stealing pontifical medals from the papal apartments.
Charged with "aggravated robbery", the devout Mr Gabriele is reported to be spending his time in quiet prayer, attending Mass and receiving visits from his wife.
He has been cast as a 21st century Judas in the whole affair, but his motives for allegedly stealing the Pope's papers remain a mystery.
"The key question is why did he do it?" said Father Thomas Reese, a Vatican expert at the Woodstock Theological Centre at Georgetown University in Washington. "Was it for money, or for political or theological reasons, or did he do it because somebody told him to?"
Swirling around the saga are accusations of espionage and betrayal worthy of an airport thriller or, as one Italian commentator said, an Agatha Christie mystery.
The leaks may be evidence of groups of bitterly opposed cardinals jockeying for position in the hope of selecting the next Pope, as Benedict looks increasingly frail.
The documents cast in a particularly bad light Tarcisio Bertone, who as secretary of state is the Pope's right-hand man and effectively the prime minister of the world's smallest state.
Regarded as overbearing, authoritarian and having far too little diplomatic experience for the job, he is held responsible for a long list of bungles which have embarrassed the Pope, from the rehabilitation of a Holocaust-denying British bishop to bizarre and confused guidelines about the Church's attitudes to condom use.
He is also resented by an old guard loyal to his predecessor, Angelo Sodano, who are bitter abut being passed over for promotion.
"There is deep disillusionment and animosity towards Bertone," a Vatican insider who asked not to be identified told The Sunday Telegraph.
"He is widely seen as an ineffective administrator. Normally Secretaries of State have a strong diplomatic background, but he doesn't. He doesn't even speak English. This pontificate has been marked by many mishaps and a lot of it is blamed on Bertone. There are a lot of people inside the Vatican who would like to see him leave."
Resentment towards the Secretary of State was further fuelled in February, when the Pope announced the appointment of 22 new cardinals – many of them considered Bertone loyalists. It is the Sacred College of Cardinals which, in a closed door conclave after the death of Benedict, will elect the next Pope.
For now the German pontiff is standing by his 77-year-old lieutenant, declaring that he still has trust in his "closest collaborators".
On Saturday the two men were together in Milan during a three-day trip to promote the institution of the family.
But the secretary of state has been badly damaged by claims in the new book – including, most sensationally, that he gave the nod to a smear campaign against the editor of L'Avvenire, the newspaper of the Italian Bishops' Conference, involving allegations of a homosexual affair and conviction for stalking - neither of which was true.
Vatican officials have denied that Cardinal Bertone did so.
The book also appears to show that Cardinal Bertone did his utmost to block efforts to clean up alleged corruption and cronyism within the city state.
Archbishop Carlo Maria Vigano, a senior member of the Vatican administration, discovered that it was wasting millions of euros in overpaying for goods and services. He found, for instance, that the Vatican had paid an exorbitant €550,000 in 2009 for the traditional "presepe" Nativity figures that are set up in St Peter's Square at Christmas, when they should have cost half that.
But Cardinal Bertone had him removed from his post three years before his term was up and sent to faraway Washington as the Pope's nuncio, or ambassador.
The book contains a pleading letter that Archbishop Vigano sent to the Pope, whom he addressed in Italian as "Beatissimo Padre", appealing in the strongest terms to be allowed to continue with his work and protesting that "transferring me at this moment would provoke profound confusion and discouragement" among those in the Vatican trying to tackle "many cases of corruption".
His protestations had no effect – he was "liquidated" by Cardinal Bertone, according to the book.
The cardinal has also been accused of impeding efforts to improve transparency within the Vatican's bank, formally known as the Institute for Works of Religion.
He was reportedly instrumental in having the head of the bank, Ettore Gotti Tedeschi, removed from his post late last month.
The secretary of state has "in recent years managed to build a spider's web of power, nominating cardinals and monsignors who are loyal to him as the heads of numerous key departments in the Vatican," Mr Nuzzi, 42, claims.
Vito Mancuso, an Italian theologist, has no doubt that powerful figures in the Vatican are out to get Cardinal Bertone. "These documents are like a series of bullets aimed at Bertone. They want to sink him, to force him to resign," he said.
None of the leaked documents paint Benedict in a negative light, but they do suggest that he has scant control over warring "princes of the Church" as cardinals are known.
The murky saga has damaged the worldwide image of the Catholic Church, just as it was trying to recover from the paedophile priest scandals.
"Faction fighting has gone on for centuries but normally behind close doors," said Father Reese in Washington. "What is upsetting to the Pope is that these divisions have been made so public."
Few people believe that the valet acted alone. It is widely speculated that he was simply "il postino" – the postman, or courier of the stolen documents, and has been put up as a convenient scapegoat.
Mr Nuzzi, the author of the explosive book, revealed last week that more than 10 separate informants fed him the stolen papers. He was first contacted in the spring of 2011, in cloak-and-dagger circumstances straight out of a John Le Carre spy novel.
He discovered later that at his first meeting with two unnamed Italian men in their forties, in a cafe in Rome, he had been under surveillance by others involved in the leaking of the documents.
"They wanted to make sure that we didn't have a 'tail', that we were not being followed," Mr Nuzzi wrote in his book, which has become a best-seller and was sold out in bookshops in Rome last week.
Those inside the Vatican have an "obsession" with secrecy, said the author, who code-named his most important informant "Maria".
Rome is abuzz with speculation that one or more senior figures, perhaps monsignors or even cardinals, had a hand in the obtaining and disseminating of the compromising documents.
Vatican investigators reportedly have another four or five suspects in their sights and are preparing to enlist the help of Italian authorities – some of the suspects are Vatican employees but citizens of Italy.
As the investigation widens, the head of the Gendarmerie may find his old contacts come in handy – Domenico Giani, 49, was formerly an agent of Sisde, Italy's domestic secret service.
Father Federico Lombardi, the Vatican spokesman, has insisted that no cardinal is under suspicion, as the Holy See reverts to its familiar tactic of denial, bluster and obfuscation.
On Saturday it emerged that a new butler, Sandro Mariotti, had been appointed to attend to the Pope's needs.
His predecessor, Mr Gabriele, was questioned again this past week by Vatican magistrates, who were expected to rule on whether there is enough evidence to send him to trial in the Palazzo del Tribunale, next to the Gendarmerie building where he is incarcerated.
If found guilty, he could in theory receive a custodial sentence, although he would have to be transferred to an Italian jail, because the Vatican has none.
"Are they really going to do that, though, when this whole affair has already proved so embarrassing?" said the Vatican insider. "It's not the Pope's style. My hunch is that they will make a deal whereby he tells them everything he knows and in return gets to keep his pension. This is not the Middle Ages, they are not going to let him rot in prison."
Nicknamed within the Vatican "Paoletto", or little Paul, he is one of a select handful of people who had daily access to the Pope in his private apartments, and was a trusted member of the pontiff's household.
His arrest, which followed the discovery of a pile of stolen documents in his Vatican apartment, was "a heavy blow", his wife said.
"My husband has always wanted the best for the Church. I am convinced that he would never have done something that could damage the Holy Father," Manuela Citti, a housewife and mother of three, told Italian journalists.
"For us in the family, but also for the many friends who have known Paolo for years, it was a heavy blow – unexpected and inexplicable. But we have confidence in the (Vatican) magistracy and we believe everything will be clarified."
Mr Gabriele, who has worked for Benedict since 2006 and previously served John Paul II, is being held within the headquarters of the Vatican Gendarmerie, the city state's 130-strong police force.
The cells are some of the least used in the world – they occasionally hold pickpockets caught in St Peter's Square but the last time all three were full was in 1971, when four employees of the Vatican's telephone exchange were accused of stealing pontifical medals from the papal apartments.
Charged with "aggravated robbery", the devout Mr Gabriele is reported to be spending his time in quiet prayer, attending Mass and receiving visits from his wife.
He has been cast as a 21st century Judas in the whole affair, but his motives for allegedly stealing the Pope's papers remain a mystery.
"The key question is why did he do it?" said Father Thomas Reese, a Vatican expert at the Woodstock Theological Centre at Georgetown University in Washington. "Was it for money, or for political or theological reasons, or did he do it because somebody told him to?"
Swirling around the saga are accusations of espionage and betrayal worthy of an airport thriller or, as one Italian commentator said, an Agatha Christie mystery.
The leaks may be evidence of groups of bitterly opposed cardinals jockeying for position in the hope of selecting the next Pope, as Benedict looks increasingly frail.
The documents cast in a particularly bad light Tarcisio Bertone, who as secretary of state is the Pope's right-hand man and effectively the prime minister of the world's smallest state.
Regarded as overbearing, authoritarian and having far too little diplomatic experience for the job, he is held responsible for a long list of bungles which have embarrassed the Pope, from the rehabilitation of a Holocaust-denying British bishop to bizarre and confused guidelines about the Church's attitudes to condom use.
He is also resented by an old guard loyal to his predecessor, Angelo Sodano, who are bitter abut being passed over for promotion.
"There is deep disillusionment and animosity towards Bertone," a Vatican insider who asked not to be identified told The Sunday Telegraph.
"He is widely seen as an ineffective administrator. Normally Secretaries of State have a strong diplomatic background, but he doesn't. He doesn't even speak English. This pontificate has been marked by many mishaps and a lot of it is blamed on Bertone. There are a lot of people inside the Vatican who would like to see him leave."
Resentment towards the Secretary of State was further fuelled in February, when the Pope announced the appointment of 22 new cardinals – many of them considered Bertone loyalists. It is the Sacred College of Cardinals which, in a closed door conclave after the death of Benedict, will elect the next Pope.
For now the German pontiff is standing by his 77-year-old lieutenant, declaring that he still has trust in his "closest collaborators".
On Saturday the two men were together in Milan during a three-day trip to promote the institution of the family.
But the secretary of state has been badly damaged by claims in the new book – including, most sensationally, that he gave the nod to a smear campaign against the editor of L'Avvenire, the newspaper of the Italian Bishops' Conference, involving allegations of a homosexual affair and conviction for stalking - neither of which was true.
Vatican officials have denied that Cardinal Bertone did so.
The book also appears to show that Cardinal Bertone did his utmost to block efforts to clean up alleged corruption and cronyism within the city state.
Archbishop Carlo Maria Vigano, a senior member of the Vatican administration, discovered that it was wasting millions of euros in overpaying for goods and services. He found, for instance, that the Vatican had paid an exorbitant €550,000 in 2009 for the traditional "presepe" Nativity figures that are set up in St Peter's Square at Christmas, when they should have cost half that.
But Cardinal Bertone had him removed from his post three years before his term was up and sent to faraway Washington as the Pope's nuncio, or ambassador.
The book contains a pleading letter that Archbishop Vigano sent to the Pope, whom he addressed in Italian as "Beatissimo Padre", appealing in the strongest terms to be allowed to continue with his work and protesting that "transferring me at this moment would provoke profound confusion and discouragement" among those in the Vatican trying to tackle "many cases of corruption".
His protestations had no effect – he was "liquidated" by Cardinal Bertone, according to the book.
The cardinal has also been accused of impeding efforts to improve transparency within the Vatican's bank, formally known as the Institute for Works of Religion.
He was reportedly instrumental in having the head of the bank, Ettore Gotti Tedeschi, removed from his post late last month.
The secretary of state has "in recent years managed to build a spider's web of power, nominating cardinals and monsignors who are loyal to him as the heads of numerous key departments in the Vatican," Mr Nuzzi, 42, claims.
Vito Mancuso, an Italian theologist, has no doubt that powerful figures in the Vatican are out to get Cardinal Bertone. "These documents are like a series of bullets aimed at Bertone. They want to sink him, to force him to resign," he said.
None of the leaked documents paint Benedict in a negative light, but they do suggest that he has scant control over warring "princes of the Church" as cardinals are known.
The murky saga has damaged the worldwide image of the Catholic Church, just as it was trying to recover from the paedophile priest scandals.
"Faction fighting has gone on for centuries but normally behind close doors," said Father Reese in Washington. "What is upsetting to the Pope is that these divisions have been made so public."
Few people believe that the valet acted alone. It is widely speculated that he was simply "il postino" – the postman, or courier of the stolen documents, and has been put up as a convenient scapegoat.
Mr Nuzzi, the author of the explosive book, revealed last week that more than 10 separate informants fed him the stolen papers. He was first contacted in the spring of 2011, in cloak-and-dagger circumstances straight out of a John Le Carre spy novel.
He discovered later that at his first meeting with two unnamed Italian men in their forties, in a cafe in Rome, he had been under surveillance by others involved in the leaking of the documents.
"They wanted to make sure that we didn't have a 'tail', that we were not being followed," Mr Nuzzi wrote in his book, which has become a best-seller and was sold out in bookshops in Rome last week.
Those inside the Vatican have an "obsession" with secrecy, said the author, who code-named his most important informant "Maria".
Rome is abuzz with speculation that one or more senior figures, perhaps monsignors or even cardinals, had a hand in the obtaining and disseminating of the compromising documents.
Vatican investigators reportedly have another four or five suspects in their sights and are preparing to enlist the help of Italian authorities – some of the suspects are Vatican employees but citizens of Italy.
As the investigation widens, the head of the Gendarmerie may find his old contacts come in handy – Domenico Giani, 49, was formerly an agent of Sisde, Italy's domestic secret service.
Father Federico Lombardi, the Vatican spokesman, has insisted that no cardinal is under suspicion, as the Holy See reverts to its familiar tactic of denial, bluster and obfuscation.
On Saturday it emerged that a new butler, Sandro Mariotti, had been appointed to attend to the Pope's needs.
His predecessor, Mr Gabriele, was questioned again this past week by Vatican magistrates, who were expected to rule on whether there is enough evidence to send him to trial in the Palazzo del Tribunale, next to the Gendarmerie building where he is incarcerated.
If found guilty, he could in theory receive a custodial sentence, although he would have to be transferred to an Italian jail, because the Vatican has none.
"Are they really going to do that, though, when this whole affair has already proved so embarrassing?" said the Vatican insider. "It's not the Pope's style. My hunch is that they will make a deal whereby he tells them everything he knows and in return gets to keep his pension. This is not the Middle Ages, they are not going to let him rot in prison."