Arsameia on the Nymphaios (Armenian: Արշամաշատ, Turkish: Eski Kale, English: Old Castle) is an ancient city located in Old Kâhta in the Turkish province of Adıyaman. The site is near Kâhtaçay, known in ancient times as Nymphaios. Arsameia was a royal seat of the kingdom of Commagene. It is best known for the Hierothesion of King Mithridates I Kallinikos, built for him by his son and heir Antiochos I.
The ancient town of Nymphaios was renamed Arsameia in the Third Century BCE by the Armenian king Arsames (255–225 BCE). It was then taken in 235 BCE by the Seleukid Antiochus Hierax who was fleeing from his brother Seleucus II, who was later claimed as an ancestor by the Commagenian King Antiochus I. The city had already been abandoned again by Roman times, stones from local graves were used by Roman soldiers or building bridges.
The Greek word Hierothesion (ἱεροθέσιον) is term for the holy burial areas of those belonging to the royal house, and is only known from Commagene. Apart from the Hierothesion which Antiochos himself built on Nemrut Dağı, and the second one on Karakuş which his son Mithridates II built for the female members of the royal house, a third is to be found in Arsameia, the burial site and the associated cultic area for Antiochus' father Mithridates. A processional way leads up the mountain in the form of a Z and passes three sites which its discoverer Friedrich Karl Dörner marked as Sites I–III. At the first of these, Site II, stands the fragment described as the Mithras Relief. It is the right hand side of a dexiosis, which shows Antiochos or Mithridates shaking hands with the sun god Mithras. Antiochus and those associated with him depicted themselves as being on the same level as the gods through these representations which are distributed throughout Commagene. Dörner was able to re-erect the upper and lower halves of Mithras, of the left-hand side of the relief only part of a shoulder was found, which Dörner however identified with one of the kings due to its clothing.