Complex sentence: The castle functioned as a toll-collecting station that was not to be ignored, as it worked in concert with Gutenfels Castle and the fortified town of Kaub on the right side of the river.
Split into simpler sentences: The castle functioned as a toll-collecting station that was not to be ignored. It worked in concert with Gutenfels Castle and the fortified town of Kaub on the right side of the river.