Complex sentence: Frewin and Rudolf escape with Antoinette's help, and when Sapt and his men arrive at the castle, Henzau switches sides and aids Frewin and Rudolf against Michael, opens the castle gates and rides away, telling Sapt that he will report for duty next week.
Split into simpler sentences: Frewin and Rudolf escape with Antoinette's and Henzau's help, just as Sapt and his men arrive at the castle. Henzau opens the castle gates and rides away, telling Sapt that he will report for duty next week.