。
The room had been kept warm for the guests.
Literal
Room [topic-は] visitors [for the sake of-のために] warm was-made-and-kept-in-advance.
Combines three layers: i-adjective adverbial 暖かく + する (make warm), ~ておく (maintain in advance), and passive ~れる (was done). The passive here describes an impersonal result — the room was prepared, with no named agent.