Complex sentence: Under King Frederick II of Prussia, also known as "Frederick the Great", the palace was once again turned into a royal residence for his wife, Elisabeth Christine of Brunswick-Bevern, who used it as her regular summer residence from 1740-90.
Split into simpler sentences: Under King Frederick II of Prussia, also known as "Frederick the Great", the palace was once again turned into a royal residence. He gave it to his wife, Queen Elisabeth Christine, who used it as her regular summer residence from 1740-90.