Merv later fell to the forces of Alexander the Great, after whose early death Hellenistic control was retained, under the Seleucids. The Seleucid ruler Antiochus I built a new city here, which he termed Antiochia Margiana. Now known as Gyaur Kala, this city was roughly square in plan, with each wall running some 2km. Its streets would have been laid out in a grid pattern. The imposing walls of the city aside, no monuments remain standing within Gyaur Kala, though there are some remnant structures of interest. The existing city of Erk Kala was retained as the citadel of the Seleucid city, and is embedded into the north wall of Gyaur Kala. From the remains of a lookout tower which form the highest point of the walls of Erk Kala, you are given an excellent view across both Erk Kala to the north of you and Gyaur Kala to your south.
Gyaur Kala and its northern citadel formed the city through the subsequent Parthian and Sasanian periods, and into the early Islamic one. Take the road from the base of Erk Kala due south, into the centre of the Gyaur Kala site, and park at the central crossroads. Just to the northeast are the hard-to-make-out remains of the 7th-century Beni Makhan Mosque, which was rebuilt in the Seljuk period. The most impressive remains here arc those of a tired-brick cistern, or sardoba. This would once have been a domed, subterranean structure, and now forms a brick-walled pit, 8m deep and 6m in diameter.
The city site of Gyaur Kala features evidence of a range of faiths, including Zoroastrianism, Christianity and Islam. Hut perhaps the most interesting religious building here is the remains of a Buddhist stupa, thought to be the westernmost Buddhist monument yet identified. The stupa sits in the southeast corner of Gyaur Kala, accessible along a track running here from the central crossroads. The stupa itself is a mound, on which the main features of the religious building are only vaguely discernible. To the south, a pock-marked terrain marks the adjacent Buddhist monastery. Among the finds made at the stupa were the beautifully decorated vase now displayed at the National Museum in Ashgabat, and a clay head of Buddha, which archaeologists believe would have been part of a massive statue, more than 3m high.
The archaeologists of the International Merv Project have excavated a cross-section through the southwestern walls of Gyaur Kala. This beautifully demonstrates how the city walls were built up through the centuries: relatively modest and narrow Seleucid walls, of around 280BC, form a core around which extensions were built upwards and outwards, reflecting the more sophisticated threats posed by advances in artillery. Several distinct further phases of wall construction can be identified, including the addition of bastions in the 4th century, which would have provided platforms for defensive artillery pieces, as well as strengthening the walls.