The Zoroastrian population is distributed between the cities and the villages. For centuries their stronghold was in the provinces of Yazd, Kermān and Fārs, but nowadays the largest populations are in Tehran, Shīrāz, and Eşfahān. The architecture of their houses has been heavily influenced by European styles, and there are no features distinguishing between the modern houses of Zoroastrians and Muslims. This was not the case in the past, however; until 1880, there were restrictions imposed on Zoroastrian architecture. Over the centuries, they had to develop architectural features that would satisfy their religious requirements while providing the security they needed against raids by fanatic Muslims. Most of the architecture that has been studied is located in the province of Yazd. Compared to Muslim houses, those of Zoroastrians used to be much smaller, and they lacked double doors, open courtyards, and wind towers—that is, tall chimneys with vents used to cool houses by creating a system of updrafts (Bonine 1980; Beazley 1977). To compensate for the lack of wind towers, they had to increase the number of air vents and holes in the roof. Roofs were restricted in height; Zoroastrians therefore built the main level of houses below ground level. Until the last two decades of the nineteenth century, two types of house were found among the Zoroastrians of Yazd, the do-pesgami and cor-pesgami, which differed in the manner in which their internal space was divided. Both types of dwellings featured a pesgam-i mas (great hall) and a pesgam-i vrok (small hall). These rooms were set aside for religious activities. (Muslim houses do not contain a room designated for religious observance.) The pesgam-i mas was never built facing the main door, in order to protect it from the eyes of those considered to be unclean according to the Zoroastrian religion. The corpesgami was much larger in size and was built in a cruciform shape (Boyce 1971a). The main characteristic found in a traditional Zoroastrian house is an open hearth in the kitchen, in which to maintain the sacred fire (the principal symbol of their religion). Clay boxes in the yard are used to sow herbs during festivals. Distinguishing features of every Zoroastrian community are the fire temples in which the holy fire is kept and members attend prayers and other religious activities. For centuries, communities concealed the location of the fire temples for fear of attacks by Muslims; fire temples were built to look like humble Zoroastrian dwellings (Boyce 1966). The restrictions imposed on Zoroastrian architecture were eliminated during the late 1920s, after the Pahlavi dynasty guaranteed Zoroastrian security. Today it is very difficult to differentiate their dwelling places from those of other ethnic and religious groups.