Yazılıkaya is the sacred counterpart to Hattusa's city ruins. Its name means "inscribed rock", and the sanctuary is known for carved reliefs showing Hittite deities in procession along the stone walls.
Do not skip it after the main site. The reliefs are among the clearest, most memorable images of Hittite religion still in place, and the short distance from Hattusa makes the two feel like one ancient landscape.