The Jameh Mosque of Semnan stands inside the old bazaar area and is the city's most important historic religious building. Its earliest fabric goes back to the Seljuk period, and tradition places it over an earlier fire-temple site, though the visible monument is firmly Islamic in form.
The brick minaret and the high western iwan are the highlights. This is not a glittering shrine; its beauty is in proportion, age, and brickwork. Visit it together with the bazaar, and dress conservatively as you would for any active mosque in Iran.