The caravanserai of Sa'd al-Saltaneh is one of the largest and best preserved urban caravansaries in Iran.
Covering an area of 6.2 hectares, the caravanserai was constructed in Qajar era during the reign of Nasereddin Shah (1831 – 1896) under the supervision of Governor Bagher Khan Sa’d al-Saltaneh for whom the caravanserai is named after. 
Located in Qazvin Province, it is built on a square plan with four porticos facing a huge courtyard. There is also a large dome accompanied by four half-domes on its sides decorated with striking tiles.
At the southern end of the yard, the caravanserai intersects with its traditional commercial chambers presenting the unique traditional architecture of Qazvin where once was the capital of the Persian Empire and contains over 2000 architectural and archeological sites. It is a provincial capital today that has been a cultural center of mass throughout history. Archeological findings in the Qazvin plain reveal the existence of urban agricultural settlements as far back as 7000 BC.
On the other hand, the caravanserai of Sa'd al-Saltaneh is considered a main commercial center and reminiscent of Iranian private sector commercial development that featured trade with Europe and Russia during the Qajar era.

More details 
The caravanserai’s main entrance opens to the Imam Street in Qazvin. A beautiful vestibule connects the entrance to the main yard of the structure that is surrounded with chambers decorated with little verandas. Behind the eastern chambers, there lies the large warehouse as well as the Sadieh Bathhouse.
Moreover, some hujrehs or the residing rooms for the travelers are situated one meter above the courtyard ground level.
The hallway behind the southern balcony has the largest dome as well as four semi-domes adjacent to it.
The eastern-western axis of the hallway is called Dalan’e Qeisariyeh or "Caesar's Hall", and the north-south axis of the caravanserai's hallway is named Dalan’e Ghahremani or "Ghahremani Hall". The later is connected to the "Bazaar of Vizir" of the city. There are also two smaller courtyards in the east and west of the caravanserai.
The historical structure lost its brisk market due to several political and economic changes over time with businessmen gradually abandoning their stores.
The caravansary was purchased by the Cultural Heritage Organization in 1999 and subsequently renovated by the Cultural Heritage Department of the province.
                                                                                           (Source: Historical Iran)