How did he ever succeed in getting to Billie?
I had locked her door, and taken away the key.

It was not until I invaded the last room on the main floor that I solved this riddle--the two apartments formed a suite with connecting door between.

However he was not there now, and all that remained to search was the servants' ell.
The hallway narrowed, and was lower by a single step, the back stairs at the left.

There was no window, and with all the doors closed, I could see down only a portion of the way.

The hallway itself was gloomy, the shade of the rear window being closely drawn.