20 B.C. BC, the Roman Emperor Augustus expanded the empire of the Cappadocian client king Archelaus I to include part of the mountainous regions of Cilicia. The acquisition of the Cilician coastline prompted Archelaus to move his residence from the Cappadocian Mazaka to the (much more pleasant and easily accessible for travellers) Cilician island of Elaiussa, where he had a palace built. Archelaus renamed the place "Sebaste" (after Sebastos, the Greek form of Augustus) in honor of Emperor Augustus. Archelaus also received state guests at his palace in Elaiussa-Sebaste, including his brother-in-law, King Herod the Great of Judea.
Well preserved are a theatre, the agora, on which a church was built in late antiquity, a Roman temple that was converted into a church, a large late antique round building with a portico at the harbour, remains of aqueducts and thermal baths. There are numerous very well preserved necropolises along the roads surrounding the city.
Source: Wikipedia