There are about 50 fire-temples in Iran, associated with the fire-worshipping Zoroastrian religion. Generally square on plan, they consist of four massive piers carrying an arch on each side (so called chahār tāq, meaning ‘four-arched”), the whole composition roofed over with a dome supported on squinches. They are likely to be of the Sassanian period. After the C7 Moslem invasion, many fire-temples were destroyed, but some were converted into mosques, and indeed some may have been the prototypes of prayer-halls.
Jane Turner (1996)