"The temple is adorned with costliest offerings consisting of early works of art."
Strabo, Geography (XIV.1.5)
The fourth largest sanctuary in the Greek world after the Temple of Artemis (and the Heraion at Samos and the Olympieion at Sicily), the Didymaion was built to rival the Artemision, employing one of the same architects, who was completing his work there, and having the same approximate dimensions.
In 334 BC, Alexander the Great liberated the cities of Ionia. Although the oracle pronounced him to be the son of Zeus in 331 BC, it probably was not until about 300 BC (the cult statue of Apollo had been returned in 301 BC) that the citizens of Miletus were able to begin rebuilding the earlier archaic temple that had been plundered (including treasures that had been dedicated by Croesus) and burned by the Persians in 494 BC. Betrayed by the Branchidae, the priestly caste who were guardians of the site, in exchange for their lives, their descendants later were said to have been massacred by a vengeful Alexander. But the project proved too ambitious and the magnificent structure never was completed. One hundred and twenty columns were planned, each over sixty-four feet high (the tallest of any Greek temple). Inscribed accounts of the construction indicate that each column cost approximately forty-thousand drachmas, at a time when a worker earned about two drachmas a day.
Suetonius indicates that Caligula intended to complete the sumptuous temple, and Pausanius (VII.5.4) later writes that the temple was unfinished even in his own time, almost five centuries later. When it was learned that the Temple of Artemis had been destroyed by invading Goths, the temple at Didyma was fortified against attack and escaped destruction. But its famous oracle, second only to that at Delphi, was silenced by the edict of Theodosius to Cynegius in AD 385 and later closed altogether, its sanctuary replaced by a church. At the end of the fifteenth century, an earthquake reduced the temple to rubble, collapsing all but three of its columns.
The temple is unusual in that it was hypaethral and had no roof. Raised on a high, stepped podium and surrounded on all four sides by a double row of columns (double peristyle or dipteral), twenty-one along the sides and ten across the front and rear façades (decastyle), the interior cella (naiskos) was exposed to the sky, providing a large open sanctuary (adyton) within this forest of columns. Behind an array of twelve more columns in the temple's deep porch (pronaos), there was a great doorway but with such a high threshold that it did not serve as an entrance but as an antechamber or stage, on either side of which were two vaulted passageways that descended in the dark, not to the traditional cella but out onto the sunlit inner court of the sanctuary, itself. There, at the far end, was a small tetrastyle temple that housed the cult statue of Apollo and a spring. Turning around, one saw a broad flight of stairs leading up to doors on the other side of the antechamber.
Having fasted for three days in preparation and inspired by the chthonian powers of the sacred underground spring, the prophetess received the oracle of the god, but it is uncertain exactly how that message was conveyed. It may be that the antechamber was used as a chresmographeion and the oracle was pronounced from there by the prophet of the temple, or it may be that the cult statue was intended to be seen and petitioners were permitted to go down the passageways to the adyton. The oracle chamber also may have been another building altogether, where the prophecies, many of which survive, were rendered into verse and delivered in writing.
In the variety and complexity of its interior spaces, Didyma is unique. It is exceptional for another reason: In 1979, fine, barely visible lines were discovered incised on the high interior walls of the adyton. They are the actual blueprint of the temple, rendered in full-scale and precisely scratched into the surface of the marble to serve as a guide over the several lifetimes it would take to complete construction. They survive at the Didymaion because the temple never was finished and the walls of the cella court did not receive their final polishing.
"With regard to the enlargement made at the middle columns, which among the Greeks is called entasis, at the end of the book a figure and calculation will be subjoined." Vitruvius' De Architectura originally was illustrated with ten such drawings, all of which have been lost. One set of inscribed lines at Didyma actually provides a scheme for laying out the entasis (the slight convex bulge of the column to make it appear straight) and so provides an example of how the missing scale drawing might have looked.
References: Didyma: Apollo's Oracle, Cult, and Companions (1988) by Joseph Fontenrose; Lothar Haselberger (1985); The Seven Wonders of the World (1995) by John and Elizabeth Romer; Greek Architecture (1996) by A. W. Lawrence, revised by R. A. Tomlinson.; Vitruvius: The Ten Books on Architecture (1960) translated by Morris Morgan (Dover Books); The Geography of Strabo (1929) translated by Horace Leonard Jones (Loeb Classical Library); Pausanius: Description of Greece (1933) translated by W. H. S. Jones and H. A. Ormerod (Loeb Classical Library).