Which architectural features of domed vaults and iwans are associated with the Parthian period?

During the Parthian period, architectural design was characterized by two significant features that later became foundational to Islamic architecture:

• The Iwan: This is a three-sided hall where the fourth side is replaced by an open archway. In Parthian sites such as Hatra and the palace at Ashur, the number of iwans often multiplied, and their adjoining facades were frequently decorated with engaged columns.1
• Domed Vaults: Parthian architecture utilized vaulted chambers, which are well-preserved in the central group of buildings at Hatra. While the later development of the pendentive allowed for more complex transitions from square rooms to circular domes, the Parthian period established the use of vaulted spaces within their sanctuary and palace designs.12

These elements reflect the architectural innovations of the era, which moved away from the columned halls of the earlier Achaemenian period toward the use of arches, vaults, and the distinctive open-hall structure of the iwan.13