Didyma was an ancient Greek sanctuary on the coast of Ionia in the domain of the famous city of Miletus. Apollo was the main deity of the sanctuary of Didyma, also called Didymaion. But it was home to both of the temples dedicated to the twins Apollo and Artemis. Other deities were also honoured within the sanctuary. The Didymaion was well renowned in antiquity because of its famed oracle. This oracle of Apollo was situated within what was, and is, one of the world's greatest temples to Apollo. The remains of this Hellenistic temple belong to the best preserved temples of classical antiquity. Besides this temple other buildings existed within the sanctuary which have been rediscovered recently; a Greek theatre and the foundations of the above-mentioned Hellenistic temple of Artemis, to name but two.
The Hellenistic temple had two predecessors. The first sacred building dedicated to Apollo was erected around 700 BC. It was probably a hekatompedos, which means 100 feet long. The width of this first sekos measured 10 meters. 'Sekos' is Greek for ‘courtyard’; we can therefore deduce that the late geometric temple and its successors had never been roofed. The oldest temple of Apollo surrounded the sacred spring and the sacred laurel tree. This spring and tree formed the centre of the sanctuary for more than 1000 years. From the middle of the 6th century BC the Milesians raised a new temple and constructed a new altar for Apollo. Alas, of this temple only the foundations of the sekos wall survived.
The entrance was at the east side. With a pronaos of three rows of four columns, the approaching visitor passed through a regularized grove formed by the columns. The door usually leading to a cella was replaced by a blank wall with a large upper opening through which one could glimpse the upper part of the naiskos in the inner court (in inscriptions the inner court is referred to as "sekos" or "adyton"). The impassable threshold of this door stands 1.5 meters above the floor of the pronaos, while the entire door reaches a height of 14 meters. The entry route lay down either of two long constricted sloping tunnels built within the thickness of the walls and giving access to the inner court, still open to the sky but isolated from the world by the 25 meters high walls of the sekos. This was the location of the oracle spring, the laurel tree and the naiskos with the cult statue. The foundations of the naiskos are 8.24 meters wide and 14.23 meters long. This Ionic prostylos was built around 300 BC. It is famed because of the superior high quality of its ornamentation. The naiskos with the cult statue of Apollo is depicted on Imperial coins of Miletus. The sacred oracle spring was not situated in the naiskos, but in the eastern half of the sekos. It was found beneath the early Byzantine church. The inner walls of the sekos were articulated by pilasters. The capitals of them are ornamented with griffins and flowers. Among them a long frieze with griffins decorated the whole sekos.en.wikipedia.org