Samarra

City · c. 836–892 CE · in Europe

Capital of Abbasid Caliphate (836–892).

Heraklion (c. 830 CE): Capital of Emirate of Crete (830–960).
Jaca (c. 830 CE): Capital of County of Aragon (830–921).
Aínsa (c. 825 CE): Capital of Kingdom of Sobrarbe (825–829).
Kashgar (c. 850 CE): Capital of Kara-Khanids (850–1045), Eastern Kara-Khanids (1046–1138), Western Karakhanid Khanate (1046–1084), and 4 more.
Palermo (c. 850 CE): Capital of Emirate of Sicily (850–1091), Kingdom of Sicily (1139–1266), Kingdom of the Two Sicilies (1815–1860).
Balasagun (c. 850–1218 CE): Capital of Kara-Khanids (850–1045), Eastern Kara-Khanids (1046–1138), Western Karakhanid Khanate (1046–1084), and 1 more.
Gangaikonda Cholapuram (c. 850 CE): Capital of Chola Empire (850–1259).
Mashhad (818 CE): Capital of Afsharid Iran (1736–1756).
Nitra (c. 814 CE): Capital of Principality of Nitra (814–839).
Sibenik (860 CE): Capital of Kingdom of Croatia (926–1098).

Around the same time and place

Al-Khwarizmi's algebra (c. 820 CE): Al-Khwarizmi's treatise on solving linear and quadratic equations systematically founded algebra as a discipline; the word 'algebra' comes from al-jabr in its title.
Abbasid Caliphate (750–1258 CE): The Abbasid dynasty, descended from Muhammad's uncle, took the caliphate from the Umayyads and ruled from Iraq, presiding over the Islamic Golden Age.
House of Wisdom (c. 830 CE): An Abbasid centre of scholarship in Baghdad that flourished under Caliph al-Ma'mun, where scholars translated Greek, Persian and Indian works into Arabic.
Palmyra (c. 261–1929 CE): Capital of Palmyrene Empire (261–269).
Harun al-Rashid (c. 763 CE – 809 CE): Fifth Abbasid caliph (r. 786–809), whose reign is remembered as a golden age of Baghdad and who appears in the One Thousand and One Nights.
Al-Khwarizmi (c. 780 CE – 850 CE): A Persian mathematician and astronomer at the House of Wisdom whose works introduced algebra and Hindu–Arabic numerals to later scholars; the word 'algorithm' derives from his name.
Paper mill in Baghdad (c. 794 CE): Papermaking reached Baghdad from Central Asia; a paper mill is recorded there around 794, making paper widely available for books and records in the Islamic world.
Al-Razi (c. 865 CE – 925 CE): A Persian physician and chemist from Rey who wrote influential medical encyclopedias and gave an early clinical description distinguishing smallpox from measles.