She didn't use the door

She appeared from time to time in my flat, always at night. I never realized when she got inside. She didn't use the doorbell. I usually met her sitting on the couch watching the TV. At first it was quite scary but over time I got used to her frightening way...
Little by little we became friends. I have to say that she was very friendly and she was always in good mood. We could talk to each other during hours. She was very cultured and knew much about history, specially about things that happened centuries ago. She could describe some facts from the past as well as if she had lived that time. However she talked only about what she wanted and never answered questions about her.
One time I asked for her origins and descent. I still didn't know if she had family. She told me that it would be much better for me if I didn't know some things. Her look became danger at times but she never stopped to smile at me.
Just as she appeared she disappeared. You could never gues when this would happen. I never gave her a key from the door. I started thinking that she used the window but this tought was not specially reasonable as I lived on the seventh floor...