It looks like what we may be seeing here is a lack of a doorknob. I think it may just be the base, which is risen in the middle. Not sure either way, however.

It's fairly simple. Picture yourself eye-level with the lower left corner of the photograph. You would be staring at the handrail. From there, you could walk forwards until you reached the wall. To your right would be six or so steps. If you were to go up those steps and turn right, it would leave you facing a bunch more of steps (twenty or more). Basically, what your looking straight at is a wall, not down or up the center of a staircase.