Complex sentence: The area of the Park was regarded as unfit for human settlement in the past when it was located at the border of Poland and Western Pomerania (later-Prussia) and only since the 17th century has its forests been settled by humans.
Split into simpler sentences: The area of the Park was in the past regarded as unfit for human settlement. In the past it was located at the border of Poland and Western Pomerania (later-Prussia) and only since 17th century its forests were settled by humans.