A 21-year-old University of Wisconsin senior was held at knifepoint in the stairwell of a North Carroll Street apartment building early Saturday morning when a man demanded her purse.

Lucy Chao said she was walking home alone from State Street when a man followed her inside the unlocked entrance door of her apartment building. He then caught the locked door to the stairwell before it closed, she said.

The man started a short conversation with her, but pulled out a knife and told her to hand over her purse when she tried to walk away, Chao added.

“It was really scary but people always think ‘it won’t happen to me,'” Chao said. “I [usually] always feel safe.”

After a University of Wisconsin student was sexually assaulted in an unlocked common area of her Randall Court apartment building last summer, the city initiated an ordinance requiring landlords to install guarded latches on doors of multi-unit apartment buildings, with enforcement beginning in July.

Ald. Austin King, District 8, said tenants of the Randall Court apartment building lobbied for a lock on the front door of the building, but their landlord refused. The lock ordinance was passed due to incidents such as these, King said.

“Obviously, safety was the primary motivating factor behind the passage of the ordinance,” King said. “It was such a strong factor that the Apartment Association [of South Central Wisconsin] endorsed the passage of this ordinance.”

According to King, city officials were “embarrassed” when they discovered that although new buildings were required to have locked front doors, there was not a law requiring locks on older buildings.

Co-owner and manager of Bill’s Key Shop Edward Scherer said regardless of new locks, many students do not lock their doors, and shortly after the ordinance, many apartment buildings may have doors “jammed open,” because students do not like the added hassle of unlocking another door.

“It’s probably for security reasons,” Scherer said about the ordinance, “but it’s going to be a financial drain on some of these apartments.”

The law requires locks on “all exterior doors into all residential buildings … where the main entrance door … is accessed from a common area.” For buildings also containing a “non-residential use,” the exterior door can remain unlocked while the area of the building is occupied.

The main entrances to some campus apartment buildings remain unlocked at all times, King said, because there are offices in the entrance to the building. The new ordinance requires a lock on the exterior door, but “while their office is open and staffed, they will be allowed to pop it open,” according to King.

Early Friday morning, a man was escorted out of a private Sigma Chi fraternity party when he pointed a small gun at the hosts of the party and escaped.

Lt. Carl Strasburg of the Madison Police Department said there are important measures students can follow to protect themselves in dangerous situations on campus.

“Be aware of your surroundings. Have a cell phone ready to call 911. Stay in a well-lit area,” Strasburg said, adding the police department also recommends taking self-defense classes.

— Aubre Andrus contributed to this article.