Exposed by a devastating earthquake in 1933, Kos’ ancient centre – an important market, political and social hub – occupies a large area south of the castle. Back in the 4th century BC, this was the first town ever laid out in blocks, and you can still discern the original town plan. Landmarks include a massive columned stoa, the ruins of a Shrine of Aphrodite, 2nd-century BC Temple of Hercules, and 5th-century Christian basilica. The site is fenced, but usually open all day.