Hatra was surrounded by two round walls, and had a large temple complex in the center. Another Parthian city Darabjerd in southern Iran, had two rock formations within its perfectly circular wall: one a castle, the other a temple.
Modern Firuzabad, known as Gor and Ardashir-Khorra (“Glory of Ardashir”) in Sasaian times, is said to have been founded by Ardashir I (AD 224-241), Gor was Ardashir’s stronghold in his revolt against the last Parthian king. The Sasanian town was a round city encircled by double walls separated by a 35 m wide ditch pierced by four axial city gates. The plan of the town was a perfect circle, 1,950 m in diameter, divided precisely into twenty sectors with radial concentric streets.
Arial photo of ruins, from Berghe, Louis Vanden
Fire Temple, from, Cultural Heritage of Fars
“A walled central inner city probably housed official buildings at the center of which stood Ardashir’s fire temple, represented by stone building known as Takht-e Neshin.”
“The building was a long rectangular structure divided into a reception area backed by residential quarters. A long entrance hall, or ayvan, led into a high domed room, the principal audience chamber which was flanked by other domed rooms that occupied a 14m wide area on the north side of the palace. To the south, a smaller ayvan led to an inner courtyard backed by a second axial ayvan. The barrel-vaulted rooms that surrounded the courtyard constitute the residential part of the complex.”
Adashir’s Palace from Herrmann, Georgina
“Ardashir’s palace is a monumental structure of rough stone and gypsum mortar, built without fortifications, after his accession. Rectangular halls in the palace were roofed with domes constructed of rubble and gypsum mortar. This exceptionally strong and resilient dome rested on a transitional zone that transformed the rectangular walls of the room into a circular base for the cupola through the use of a small arch, the squinch, built above the four corners of the walls. This modest invention was eventually elaborated, and magnificently embellished with colored mosaics and tiles, in the later domes of Islamic Iran.”
References: Spiro Kostof, The City Shaped, 1991,
Excerpts from The Near East in Late Antiquity: The Sasanian Empire, University of California, Berkeley