そして彼女は窓際の壁を背に床に座り込む。 - And then she sat against the wall to the side of the window.
Judging the meaning, I think I managed to grasp what it means but I'm confused about how this sentence works.
I thought of this literal translation of what I think of this sentence
And then to her back she sits into the floor
- I think that the に in 背に an indirect object marker
- I think that the 2nd に is the location marker
- I can't really think of how 壁を座り込む works...