The Temple of Apollo is the reason Didim belongs on a travel map. Ancient Didyma was not a city in the usual sense, but a sanctuary and oracle tied to Miletus by a sacred road. The Hellenistic temple was planned on a gigantic scale and never fully finished, which somehow makes the ruins more impressive: stairways, wall passages, carved bases, and standing columns reveal the ambition without smoothing it into a tidy monument.
Come early or late if you can. The site is compact, but the stone holds heat, and the best way to read it is slowly, looking down into the open inner court and up at the surviving columns.