Qazvin contains several archeological excavations dating back 9000 years ago. There are also 23 castles from the Ismaili Assassins nearby as well. And in the middle of the city, there lies the ruins of Meimoon Ghal'eh, one of several Sassanide edifices in the area.

Qazvin contains few buildings from the Safavi era when it was capital of Persia. Perhaps the most famous of the surviving edifices is the Ali Qapu mansion, today a museum in central Qazvin.

Historical Mosque
After Islam, the abundant attendance of mystics (ascetics), as well as the prevalence of tradition (Hadith), religious jurisprudence (Fegh´h), and philosophy in Qazvin, led to the emergence of many mosques and religious schools among which the most magnificent ones are:

Jame e Atigh Mosque: One of the oldest mosques in Iran constructed by the orders of Harun al-Rashid in 807AD. In spite of the devastating Mongol invasion, this mosque still stands today in its full glory.
Heidarieh Mosque: Renovated by Amir Khomär-täsh after the earthquake of 1119 AD, the history of construction of this mosque goes back to pre-Islam, where it was a fire temple.
Masjed Al-nabi (Soltani Mosque): With an area of 14000m², this mosque is one of the most glorious mosques of antiquity, built in the Safavid period.
Sanjideh Mosque: Another mosque of Qazvin dating back to pre-Islamic Iran; a former fire temple. Its present day form is attributed to the Seljukian era.
Panjeh Ali Mosque: A former place of worship for royal harem members in the Safavid period.
Peighambarieh School-Mosque: Founded 1644 according to inscription.
Molla Verdikhani School-Mosque: Founded in 1648.
Salehieh School-Mosque: Founded in 1845.
Sheikhol Islam School-Mosque: Renovated in 1903.
Eltefatieh School: Dating back to the Il-Khanid period.
Sardar School- Mosque: Made by two brothers Hossein Khan and Hassan Khan Sardar in 1815, as a fulfillment of their promise if they came back victorious from a battle against the Russians.

Churches and Russian architecture
Qazvin actually contains three buildings built by The Russians in the late 19th/early 20th century. Among these is the current Mayor's office (former Ballet Hall), a water reservoir, and the Cantor church where a Russian pilot is buried.

According to explorers Pietro Della Valle (1588-1713), Jean Baptist Tavenier (1605-1689), Johannes Chardin (1643-1713), and others, there have been many Christians of various sects living in Qazvin for centuries. Qazvin is where The Saint Hripsime Church is located, and it is also where four Jewish prophets gave tidings of the arrival of Jesus Christ. Their tomb is now a popular shrine called Peighambariyeh.

Castles and forts
These are castles and fortifications left over mostly from the Isma'ili movement of the middle ages:

Alamout Castle
Lambesar Castle
Shirkouh Castle
Qez Qaleh Castle
Shemiran Castle
Meimoon Ghal'eh
Barajin Qaleh

Tombs, Shrines, and Mausoleums 
The Kharaghan twin towers, built in 1067 AD, Qazvin province.Another grand attraction in Qazvin Province, is the tombs of two Saljuki era princes, Aboo Saeed Bijar son of Sad and Aboo Mansoor Iltai son of Takin, that are located in two separate towers known as the Kharaghan twin towers. Constructed in 1067, these are the first monuments in Islamic Architecture which include a non-conic two-layered dome.

Unfortunately, both towers were severely damaged by a devastating earthquake on March 2003.

Some popular shrines and Mausoleums in Qazvin province are:

Imamzadeh Hossein
Peighambarieh (where 4 Jewish prophets are buried)
Imamzadeh Ismail
Ameneh Khatoon
Zobeideh Khatoon (that has also a unique traditional water reservoir)
Imamzadeh Abazar
Imamzadeh Abdollah and Imamzadeh Fazlollah in Farsajin
Imamzadeh Vali in Ziaabad
Imamzadeh Kamal in Ziaabad
Imamzadeh Ali in Shekarnab
Haft Sandoogh Pilgrimage Place
Tombs of Hassan Abad and Shahkouh
Soltan Veis
Mausoleum of Pir e Takestan
Kafar Gonbad
Hamdollah Mostowfi's Tomb
Imam Ahmad Ghazali's Tomb
Molla Khalila's Tomb
Shahid Sales' Tomb
Raeesol Mojahedin's Tomb
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Traditional Reservoirs
In the old days, Qazvin was nicknamed the 'city of water reservoirs'. Of the 100 or so water reservoirs of Qazvin, only 10 remain today, all protected by the Provincial Cultural Heritage Organization. See: List of famous ab anbars of Qazvin

Bazaars and Caravanserais
Qazvin has some fine examples of centuries old Bazaars and caravanserais:

Sa'd-ol-Saltaneh Complex
Qeisarieh
Saray e Vazir
Saray e Razavi (Shah)
Saray e Hadj Reza
Sadieh Bazaar
Shah Abbasi Caravanserai of Avaj
Shah Abbasi Caravanserai of Mohammad Abad
Hajib Shah Abbasi Caravanserai (Keikhosro)
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Old City Gates and other popular edifices
 
Peihgambariyeh ("the place of the prophets"). The mausoleum contains the resting place of four Jewish saints.During the 9th century A.D. seven gates made entrance to the city possible. In Qajar period there existed nine gates surrounding the city which were connected to each other through a wall around the city. These gates (darvāzeh in Persian) were:

Panbeh Riseh
Sheikh Abad
Rasht
Maghlävak
Khandaghbar
Shahzadeh Hossein
Mossala
Tehran
Räh e Koushk
Due to 20th century hasty urban expansion, unfortunately only the last two gates remain standing. Other popular attractions of Qazvin province include:

Chehel Sotun Museum-Palace
Hosseinieh Aminiha, a fine example of Traditional Persian residential architecture indigenous to Qazvin.
Shah Abbasi Bridge
Safa Traditional Bath
Qajar Traditional Bath

Famous Qazvinis 
craftsmanship at Shazdeh Hosein shrine.Aside from Shahzadeh Hossein, a shiite saint, where a handsome shrine has been built, there are an abundance of scientists and mystics who lived in Qazvin, or came from Qazvin, or whose tombs are scattered throughout the cities and villages of the province. Some of these are:

Ali Akbar Dehkhoda: Prominent linguist and author of Iran's first modern Persian dictionary, was originally from Qazvin.
Obeid Zakani
Oveis Qarani: A celebrity of early Islam, thought to have been killed here while fighting against an army of Deilamian origin. His tomb is known as Sultan Veis.
Hamdollah Mostowfi: The great Il-Khanid historian and writer (1281-1349) and author of The Selected History (Tarikh Gozideh), Nezhatol Qoloub and Zafar Nameh. The turquoise conic dome and its inscription in Sols calligraphy in which Mostowfi’s family tree and his works are introduced are the features that distinguish the tomb from other historical monuments of Qazvin.
Imam Ahmad Ghazali: Famous Iranian Gnostic who died in 1126 CE and was buried beside Shahzadeh Hossein. His tomb up to the end of the 16th century A.D. became the pilgrimage place for mystical sects. Following Shah Tahmasb’ s stubborn policies against philosophers and mystics and destroying Ghazali’s tomb, a group of his disciples took the remains of his body to the present place in Imamzadeh Ismail alley and constructed a new mausoleum for him. The monument was destroyed again in Mohammad Shah Qajar’s period only to be re-constructed by Majdol Islam Qazvini in 1910. Beside Ghazali’s tomb there is another tomb belonging to Soltan Seyed Mohammad Vali which dates back to 1625 CE
Molla Khalil Ibn Ghazi Qazvini: Famous faghih (religious jurist) and famous commentator of the Qur'an in Safavid period (d 1678).
Shahid Sales: Killed in 1846.
Ra'ees ol-Mojahedin: The late Mirza Hassan Sheikhol Islam son of Mirza Masood Sheikhol Islam, leader of the liberals and constitutionalists of Qazvin whose endeavors and devotion to abolish the Qajar dynasty and conquer Tehran brought the title of Raeesol Mojahedin (chief of fighters) for him.
Ali Ibn Shäzän
Ibn Majeh
Kheirol Nesaj
Ibrahim Estanbeh Heravi
Razi-olddin Taleghani
Noor-olddin Geeli
Ali Ibn Ghazi Ibn Ahmad
Imam-olddin Rafee
Siah Kolah
Vaez Qazvini
Allameh Zarabadi
Sheikh Alak Qazvini
Davoud Ibn Soleiman Ghazi
Pir e Sefid
Pir e Alamdar
Molla Abdolvahab Darolshafaee
Mohammad Ibn Yahya: Commentator of Qamoosol Loghat