The time when Hogwarts Castle's gates were locked was determined by sunset, changing with the four seasons of spring, summer, autumn, and winter. There was no fixed time.
After the gates were closed, neither visitors nor people inside the castle could come and go as they pleased. However, students below fifth year could move about freely until nine in the evening. They could study in the library or find a corridor to chat and play in, but this rule changed after Harry enrolled.
In the first year, the third-floor corridor became a forbidden area, and no one was allowed near it. In the second year, the Chamber of Secrets was opened, and nowhere except the common rooms was safe. In the third year, because of Sirius Black, after he failed to enter the Gryffindor common room, students from every house slept in the Great Hall, as though even the common rooms were no longer safe.
Then came the fourth year.
Pomona hiccupped as she headed toward the small underground room Neville had discovered. At last, this year had neither forbidden zones nor Basilisks, Dementors, or escaped prisoners entering the school, but old Barty Crouch had died. Though he had not died on campus, it had happened by the edge of the Forbidden Forest.
"You have no evidence to accuse Severus Snape of being connected to Barty Crouch's murderer." In the headmaster's office, Albus told the Aurors from the Ministry of Magic in an authoritative tone, "He was at school on the day of the murder. After the gates were locked, it would have been impossible for him to leave."
"But we have reason to believe that Hogwarts has secret passages leading outside. Perhaps Professor Snape knows some of them." Kingsley Shacklebolt looked at Snape, who stood silently nearby. "Given how