After the division of the Roman Empire in 395 AD, Aegium became a part of the Eastern Roman Empire and later the Byzantine Empire. Later, in 805 it was captured by Slavic tribes during the Slavic invasions. Probably between 9th and 13th century it was renamed to Vostitsa (Βοστίτσα). In the "Chronicle of the Morea" (14th century), it is first mentioned with this new name. The following centuries the name is found in various documents having different spelling forms like "Bostizza", "Ligustizza", "Bostitza", "Bostiza", "Vostizza".