|Beyond the southern eivan of the mosque and set amongst a labyrinth of Seljuk brickwork cloisters we find the oldest extant part of the mosque, surmounted by the austere and lofty dome of Nizam ul-Mulk, the vizier of Malek Shah.
The differences in surface treatment between the zone of transition of the brickwork dome and the rough plaster of the supporting piers lends support to the view that when Nizam ul-Mulk rebuilt the sanctuary as testified to by the inscription at the base of the dome, he actually only replaced the dome, possibly following a fire, on top of a much older construction. Under this view the lower part of the sanctuary, comprising doubled or coupled piers is typical of pre-Seljuk, possibly even Sassanian, architecture. You can see the difference in the curvature of the lower arches compared with those in the zone of transition beneath the main dome and this is generally characteristic of an earlier style. Note too the use of the broken arch to enclose an unbroken one, for example in the archway, more or less in the centre of the illustration above. This purely decorative feature, is typically Persian.
Both this dome and the chamber of Taj Al-Mulk beyond the northern eivan were probably undertaken by the same architect, however, whereas the latter is characterised by its very homogeneity, this one is characterised by its differences, which extend even to the brickwork of which the upper and lower parts are constructed.
There is, additionally, a strong local tradition that the 10th Century Sahib Ismail ibn-Abbad, who was responsible for the mosque which originally stood on the site of the Masjed-e-Hakim, used to teach in this sanctuary, and the Sassanid style of the columns has probably contributed to the belief that there was once a Zoroastrian fire temple on the spot.
Before leaving the chamber, it is worth looking at the labyrinthine kufic plaque on the north-eastern wall. The construction of the outer skin of the dome is best observed from the rear of the mosque where there is a small continuation of the bazaar.