constructed of fine but loosely mortared café-au-lait brick

On the 13th of November 1933, Robert Byron visited a bunch of monuments in the area of Damghan. One of these is the Gunbad-i Chihil Dukhtaran, that he records in his travelogue together with the other tomb tower he sees, the Gunbad-i Pir-i ‘Alamdar.

Reading the short paragraph Robert Byron writes about the two tomb towers we can get that he was not impressed by the two constructions.

The Gunbad-i Chihil Dukhtaran is a brick tomb tower that dates back to 1054-55. It is one of the oldest remaining tomb structures from the time of Tughril Beg (1040-1063), the first Seljuk monarch. The structure of the tomb tower is quite plain: a cylindrical chamber, covered by an elongated dome.

The decoration of the external surface is concentrated around the entrance and in the upper level of the building. Six decorative bands run around the surface just below the dome. These include two decorative bands ornated with swastikas and triangular elements, that frame the inscription band, in Kufic script. The inscription includes the name of the patron of the tomb: Amir Abu Shuja Asfar. The name of the patron is inscribed also on the arch framing the entrance.

Internally, the surface of the monument is covered with plaster, with no decoration.

Not shockingly beautiful: Robert Byron was probably right. Still, the Gunbad-i Chihil Dukhtaran deserved a stop in the journey.

Sources
Further images and information can be found on archnet.
The inscriptions have been transcripted and translated by Sheila Blair: S. S. Blair, The Monumental Inscriptions from early Islamic Iran and Transoxiana, Brill, Leiden 1992.