Complex sentence: The temple fell into disrepair over the next four hundred years, before, in the mid-thirteenth century, Emperor Gosaga and his son Emperor Kameyama turned the area into an imperial villa they christened.
Split into simpler sentences: The temple fell into disrepair over the next four hundred years. In the mid-thirteenth century, Emperor Go-Saga and his son Emperor Kameyama turned the area into an imperial villa which they christened.