Why had Defarge kept the door to Dr. Manette’s room locked? (Book 1 Chapters 1-6)

Defrange takes the doctor home from the Bastille. Dr. Manette is hardly in good shape and has little control over his senses. Because of the doctor’s poor condition, he cannot be allowed to wander about Paris, which has changed since his imprisonment, for fear that he will get lost. The door is locked to prevent Dr. Manette from wandering.