Apart from an infinite number of windows, my house has nineteen interior doors. Doors are not really necessary in a dollhouse, and they take a lot of space, but they certainly add to the natural look. In theory, your dolls should be able to walk from any room to any other room.
Doors are almost as boring to paint as windows, but you can so some interesting thing with them. Some of mine will be stained rather than painted (or so I think at the moment). Some may be other that white. I have distressed the four doors in the basement by painting a thin layer of green on top of the white and wiping it off immediately with a cloth.
To look natural, doors need door knobs. I am not going to pay a fortune for door knobs from dollhouse suppliers, and I have already tested some intresting methods in my previous projects. I had a vague idea what I wanted for the basement doors, and as I rummaged in my nail och screw cabinet, I saw a perfect solution.
I found these brass screws that I have no idea where they come from (and that possibly cost more than dollhouse door knobs). I bit off the top of the screw, drilled a hole in the door and fixed two knobs on each side. This is what it looks like in its environment - the door frame still unpainted.
I will have the same door knobs for all doors in the basement, but I have many other ideas for door knobs that I will implement. Luckily, there are so many doors in the house!