AB: "Numerous major events and personalities of historic importance have been associated with Merv. These include Abu Muslim, who initiated the Abbasid revolution..."

initiated under the reign of al-Mo'tasem the Abbasid (AB ev)

Great Mosque of Sousse ... Built in 851 by the amir Abu al-Abbas Muhammad al-Aghlabi of the Aghlabid Dynasty, an Abbasid Caliphate vassal (wiki)

Great Mosque of Shibam (The presence of red baked bricks, typical of ninth century Abbasid construction, point to reconstruction efforts during the reign of Caliph Harun al-Rashid) - see link
See archnet.org

Ibn Tulun Mosque (870-879), the oldest surviving mosque in Egypt. Its minaret was modelled after the one in Samarra.
See www.sacred-destinations.com

On 25 August 750, the Abbasids, having already beaten the Umayyads in the Battle of the Zab in Iraq, conquered Damascus after facing little resistance. With the heralding of the Abbasid Caliphate, Damascus became eclipsed and subordinated by Baghdad, the new Islamic capital.
See en.wikipedia.org

Tomb of the Samanids (early 10th century)

Its capital from 836-892, several surviving buildings including the spiral minaret