Vatican City has its own distinct policing organizations and criminal justice system. A large part of its population is made up of its main military force, the Pontifical Swiss Guard Corps. The Swiss Guard mainly looks after the safety of the Pope, and its ranks are made up of Swiss Roman Catholic males between the ages of 18 to 30 who enlist in this service (via The CIA Factbook). Then there is the Gendarmerie Corps of Vatican City, a supplemental police force, which provides added security, handles traffic concerns, and conducts investigations.
According to Slate, Vatican City has only one jail and that’s just used to hold prisoners before a trial. All trials are handled by judges, not juries, and its judicial branch has three levels. First, the accused appears before a trial judge, then he or she can appeal to the Tribunale (made up of three judges), and then to its Supreme Court of Appeals.
Vatican City and Italy have a special treaty on legal matters, and some cases are given over to the Italian authorities, including crimes in St. Peter’s Square, according to the Archdiocese of Baltimore. Italy also handled the trial of Mehmet Ali Ağca, who tried to assassinate Pope John Paul II in 1981. Ağca also served the first part of his sentence in an Italian prison.