Bringing an Outside Cat Inside?

I am taking in my neighbor's 4 year old Siamese female cat. I have been taking care of her for the last 2 years as the owner didn't seem to care if she was being fed or kept warm in the winters. I finally asked her if I could keep her and she said yes, my problem is that I want to keep her inside from now on. She will stay in during the day, but wants out at night to roam. She is fixed so I don't have to worry about that.

I could use some suggestions on how to keep her in. She will sit at the door and cry to be let out and won't stop till we let her out.