Wait Until Dark

A woman named Lisa (Samantha Jones) takes a flight from Montreal to New York City, smuggling bags of heroin sewn inside an old-fashioned doll. When she disembarks, Lisa becomes worried upon seeing a man watching her and gives the doll to a fellow passenger, professional photographer Sam Hendrix (Efrem Zimbalist Jr.), for safekeeping. Lisa is roughly escorted away by the man. Later, when Lisa calls Sam about the doll, he and his wife Susy (Audrey Hepburn), who is blind from an auto accident, are unable to find it.
A few days later, con artists Mike Talman (Richard Crenna) and Carlino (Jack Weston) arrive at Sam and Susy's apartment, believing it to be Lisa's. Harry Roat, Jr. (Alan Arkin), the man who met Lisa at the airport, arrives to persuade Talman and Carlino to help him find the doll. After the con men discover Lisa's body hanging in a garment bag, Roat blackmails them into helping him dispose of the body. Roat also offers to cut them into the drug profits if they help him find the doll. Susy comes home briefly and is observed by the men, who stay silent and undetected.
The next day, Roat sends Sam on a wild goose chase photography assignment to Asbury Park, New Jersey. Once Susy is alone, the criminals begin an elaborate con game: Mike poses as a friend of Sam's, Carlino poses as a policeman, and Roat poses as an old man and then his son. Using first an innocuous story about Sam and the doll, then a darker one implying that Lisa has been murdered and that Sam will be suspected, the men persuade Susy to help them find the doll. Mike gives her the number for the phone booth across the street as his own after falsely warning her of a police car stationed outside.
During this time, Susy grows suspicious of Carlino and Roat. Gloria (Julie Herrod), a girl who lives upstairs and sometimes enters Susy's apartment without her noticing, sneaks in to return the doll and tells Susy that there is no police car outside. Wanting to confirm Susy's suspicions, the two agree to have Gloria watch the phone booth and send a signal if it's in use.
On Carlino's next visit, after he calls Roat at the phone booth, Gloria sends Susy the telephone signal, and she sends the signal a second time after Susy calls Mike to tell him she has the doll. Finally realizing that Mike, too, is a criminal, Susy hides the doll. When he walks in with Carlino and Roat following quietly, she tells him that the doll is at Sam's studio. The three leave after Roat cuts the telephone cord.
While Roat and Mike walk to Sam's studio to look for the doll, Carlino stays behind to stand guard outside the apartment building. Susy sends Gloria to the bus station to wait for Sam. When Susy discovers that the telephone cord has been cut, she prepares to defend herself when the men return by putting the criminals in the dark along with her, breaking all the bulbs in the apartment's light fixtures except for the one in Sam's photography 'safe light'. She also pours a chemical into a bowl. When Mike returns, he realizes that she knows the truth and demands the doll, but she refuses to cooperate. Mike has spent more time than the others with Susy, and he has come to admire her for her quiet strength and ability to stand up to the three of them, despite her disability. He admits to her that he and his confederates are part of a criminal plot and that Sam, as Susy suspected, is innocent of any involvement, while Roat is a particular danger. Mike assures Susy that she does not need to worry about Roat, as he has sent Carlino to kill him. However, having anticipated their plan, Roat has killed Carlino instead, and, as Mike prepares to leave, Roat stabs him in the back.
Intent on acquiring the doll, Roat chains the door shut in the dark apartment, pours gasoline on the floor, and sets a piece of newspaper on fire. Susy finally agrees to give him the doll, and he puts out the fire. Susy throws the chemical at Roat's face and desperately unplugs the 'safe light' while Roat throws his knife at her, which misses and embeds itself in the wall as the apartment is plunged into darkness. Roat lights a match but hastily puts it out when Susy, having found the gasoline, starts splashing it in his direction and onto him. But the battle ends when Roat obtains light by opening the refrigerator, whose door he props open with a rag in the hinge. Susy, hearing the refrigerator come on, and weeping as she realizes that she has lost the battle, pulls the doll out from its hiding place and hands it to him. While Roat cuts open the doll and gloats over the treasure inside, Susy is able, unnoticed by him, to arm herself with a large kitchen knife. Roat then pushes Susy towards the bedroom and pins her to the wall (presumably to rape her). Susy stabs Roat who collapses, seriously wounded. She flees but is unable to escape through the chained door. She stumbles across the floor toward the kitchen window to scream for help, but Roat unexpectedly leaps up from the floor and grabs her ankle. Screaming, Susy wrenches free, but the dying Roat doggedly pursues her, using the knife with which she stabbed him to drag himself across the floor. Susy conceals herself behind the refrigerator door, as she is unable to close it. Just as he stands to stab her, Susy finds the refrigerator cord and unplugs it. Then screen is black and we hear Susy scream. the police arrive with Sam and Gloria. Sam finds light bulbs and plugs in a few lights. Susy is unharmed behind the refrigerator door. She walks to and embraces Sam, crying. Roat is dead.

Question: How does Roat keep apartment door shut?
Answer: with a chain