Complex sentence: The Imperial villa at Mediana is an important archaeological site located close by; in 364, it was the site where Valentinian and Valens divided the Empire and ruled as co-emperors.
Split into simpler sentences: The Imperial villa at Mediana is an important archaeological site located close by. In 364 Mediana was the site where Valentinian and Valens met and divided the Roman Empire and ruled as co-emperors.