Na'in's Jameh Mosque is one of the oldest mosques in Iran, with an early hypostyle plan, brick arcades, a fine stucco mihrab, and a basement chamber linked to underground water channels. It feels spare and ancient, especially if you have just come from the grand Safavid tilework of Isfahan.
Look closely at the brickwork around the courtyard and the carved stucco near the prayer hall. The mosque's lack of a grand iwan or dome is part of what makes it so valuable: this is an earlier architectural language.