FILE - This Nov. 23, 2007 file photo shows Pope Benedict XVI leading a session of prayer and reflection with the College of Cardinals, at the Vatican. Pope Benedict XVI promised his "unconditional reverence and obedience" to his successor in his final words to his cardinals Thursday, Feb. 28, 2013, a poignant farewell before he becomes the first pope in 600 years to resign. AP Photo/Christophe Simon, pool, file People are starting to show up in the square in Castel Gandolfo to await the Pope's arrival, but they are still outnumbered by members of the media. Tony Hicks, AP Regional Photo Editor Europe & Africa - Twitter http://twitter/com/hicksy663 The pope's retirement means his famous Swiss Guards get a few days off before they have to protect the new guy. Stern-face and standing as erect as the halberds they grasp, Swiss Guards rarely betray emotion on duty. But their storied history has its early roots in a bloody drama. Nearly five centuries ago, 147 Swiss Guard died while protecting Pope Clement VII in his frantic dash to safety when Emperor Charles V's soldiers sacked Rome. A few decades earlier, the Renaissance pope, Julius II, had asked Switzerland to supply the Vatican with soldiers because he was so impressed by the courage of Swiss mercenaries. The Swiss Guards will go off-duty Thursday evening at 8 p.m. - the exact moment when the man they serve, Pope Benedict XVI, resigns. Frances D'Emilio - Twitter http://twitter.com/fdemilio The last pope to resign was Pope Gregory XII, who stepped down in 1415 to end the Great Western Schism, a nearly 40-year split over leadership of Christendom. The disagreement was a major political struggle since the Church played a central role in politics, art and daily life in much of Europe, which was slowly transitioning into the Renaissance. During that era: Heretics were burned at the stake, including Bohemian-born Jan Hus, a priest considered an inspiration for the Protestant Reformation, in 1415. Joan of Arc, who fought for France in the Hundred Years War against England, died at the stake in 1431. The Medicis were building the banking empire that would turn them into a political dynasty, make them influential art benefactors and eventually produce four popes, the first in 1513. Early Renaissance artists like Donatello were playing with perspective in sculptures of saints that adorned his country's ornate churches. Update for the Twitter world: The Vatican says retiring Pope Benedict XVI will send his last tweet as pope around 5 p.m. Rome time (1600 GMT, 11 a.m. That's also about the time he's set to leave the Vatican by helicopter to the papal retreat at Castel Gandolfo. Vatican spokesman the Rev. Federico Lombardi says the pope's Twitter handle (at) Pontifex will then be "in abeyance" until the next pontiff is selected. He says it'll be up to the next leader of the Catholic Church as to whether or not he will use it. Joji Sakurai - Twitter - http://twitter.com/jojisakurai Australia's Cardinal George Pell, one of those who will be voting for the next pope, is speaking in Rome about Pope Benedict XVI's retirement: He was well aware that this was a break with tradition, slightly destabilizing. But he felt that because of his weakness and sickness, which was only too evident today, that he just didn't have the strength to lead in church in these demanding times. Victor Simpson, AP's Rome bureau chief, who has covered the Vatican for decades. Everyone looks forward to retirement for one reason or another. In Pope Benedict XVI's case, it might be having more time for walks. Benedict took daily strolls through Vatican City's gardens. Now, awaiting his first months in retirement at Castel Gandolfo is a splendid expanse of manicured lawn, dotted by geometrically-shaped bushes for his afternoon walks. The Holy See got a good deal on the castle and its grounds. It was acquired in the late 1500s in return for an unpaid debt owed by Italian nobles. But the estate didn't always appear so beautiful. For years, it was almost abandoned after the fall of the Papal State in 1870, as modern Italy took shape. One-hundred and fifteen cardinals will be voting in the next few weeks to choose Pope Benedict's XVI successor. Here's a look at some possible contenders: Cardinal Angelo Scola, archbishop of Milan Cardinal Christoph Schoenborn, archbishop of Vienna Cardinal Gianfranco Ravasi, head of Vatican's culture office Cardinal Peter Kodwo Appiah Turkson of Ghana, head of the Vatican's justice and peace office Cardinal Odilo Pedro Scherer of Sao Paulo Cardinal Jorge Mario Bergoglio of Argentina Cardinal Leonardo Sandri of Argentina Cardinal Marc Ouellet of Canada, head of the Vatican's office for bishops Cardinal Timothy Dolan of New York Cardinal Antonio Tagle, the archbishop of Manila Brian Murphy, AP's Dubai bureau chief. Pope Benedict XVI, his dress soon to be limited to a simple white robe, has worn a ceremonial red cape for his final meeting with the College of Cardinals. The short cape with ermine trim, known as a mozzetta, is worn for special occasions. And special this occasion was, coming just hours before Benedict becomes the first pope in 600 years to resign. He looked out at the cardinals and said: "Among you is the future pope." And he promised his "unconditional reverence and obedience." Victor L. Simpson, AP's Rome bureau chief. The pope is leaving with a veiled warning to the men who will choose his successor: Work together. In his final audience with the cardinals - the so-called "princes" of the Catholic Church, Pope Benedict XVI urged them to set aside their differences as they elect the next pope. He says the College of Cardinals should be unified so it works "like an orchestra" where "agreement and harmony" can be reached despite diversity. The Vatican in recent years has been famed more for its disharmony, with the pope's own butler leaking papal papers that showed feuds and intrigue at the top of the Vatican bureaucracy. Benedict says he'll pray for the cardinals in coming days as they vote on his successor. Nicole Winfield - Twitter http://twitter.com/nwinfield Among you is also the future pope, whom I promise my unconditional reverence and obedience. Pope Benedict XVI, in his final audience to his cardinals. The Clementine Hall where the pope greeted cardinals for the last time, pledging "unconditional reverence and obedience" to his successor, is a grand 16th century room built by Pope Clement VIII in honor of Pope Clement I - the fourth pope. Covered in ornate marble tiles and Renaissance frescoes, it's basically the pope's reception room - the place where he receives VIPs from around the world. It's also the place where the body of the pope lies in state for private respects by Vatican officials. Joji Sakurai - Twitter http://twitter.com/jojisakurai The pope has promised "unconditional reverence and obedience" to his successor. Pope Benedict XVI made the pledge as he bade farewell to his cardinals at the Vatican this morning. He also left with a plea for the College of Cardinals to unite and work together "like an orchestra" where "agreement and harmony" can be reached. The cardinals, who will be voting later this month to choose his successor, are lining up and kissing his hand in farewell. Officials are expecting an enthusiastic welcome from the faithful in Castel Gandolfo, the scenic town where Pope Benedict XVI will spend his first post-Vatican days and make his last public blessing as pope. Fitting for a man looking for a quieter lifestyle, the numbers won't compare to his hectic send-off from St. Peter's Square on the eve of his retirement. Some 150,000 people flooded the piazza for his final speech as pontiff, with many others watching on giant TV screens set up along the main boulevard outside. The square in Castel Gandolfo is many times smaller - though several thousand are expected to crowd in. In betting-mad Britain, bookmakers have been busy taking bets on who will replace Benedict XVI since he announced his retirement earlier this month. The favorite is Ghanaian Cardinal Peter Turkson, who would be the first African pope. He's the front-runner at bookies William Hill, at 5-2, and Ladbrokes, at 11-4. Other leading contenders include Italian cardinals Angelo Scola and Tarcisio Bertone and Cardinal Marc Oullet of Canada. And for those wanting to gamble on a longshot, Ladbrokes has 500/1 odds on former British Prime Minister Tony Blair - a Catholic convert from Anglicanism - and Irish bookmaker Paddy Power is offering 1,000/1 on U2 lead singer Bono. Jill Lawless - Twitter http://twitter.com/JillLawless Cardinals are kissing Pope Benedict XVI's hand as they bid him farewell. Pope Benedict XVI has just met with his cardinals in the Vatican this morning before heading off toward retirement later in the day. He made brief remarks to them. More on what he said in just a bit. How does the Catholic Church even get a new pope? Well, the current one either dies or resigns. Then the church holds a papal conclave and cardinals under the age of 80 vote on who they want to lead them. This time around, 115 cardinals will be voting. The conclave begins with the cardinals in their red cassocks filing into the Vatican's Sistine Chapel, chanting the "Litany of Saints." Then they place their hand on the Gospel and promise to observe absolute secrecy during and after the conclave. They also vow to vote independently - a good way to guard against external interference. During the conclave, the cardinals live in a Vatican hotel and have no contact with the outside world: no phones, no newspapers, no tweeting. On Day 1, only one round of balloting is held; after that, the cardinals cast two votes in the morning and two in the afternoon until one man has a two-thirds majority. The outside world only knows what is going on by seeing smoke from the Sistine Chapel each time the ballots are burned. Black smoke means no decision, white smoke means a pope has been chosen. Soon afterward, the thousands of faithful in St. Peter's Square will hear two Latin words announced from the balcony: "Habemus Papam! We have a pope!  Nicole Winfield - Twitter - http://twitter.com/nwinfield