Olbia

City · c. 866 CE · in Europe

Capital of Giudicato of Gallura (866–1209).

Amol (c. 866 CE): Capital of Zaydi Alid dynasties (866–928), Ziyarid dynasty (936–979).
Porto Torres (c. 866 CE): Capital of Giudicato of Logudoro (866–1259).
Smolensk (863 CE): Capital of Principality of Smolensk (1056–1406).
La Seu d'Urgell (c. 870 CE): Capital of County of Urgell (870–1413).
Brunswick (861 CE): Capital of Duchy of Brunswick (1815–1870).
Zaranj (c. 861 CE): Capital of Saffarid Dynasty (861–899).
Sibenik (860 CE): Capital of Kingdom of Croatia (926–1098).
Beiting (c. 860–1138 CE): Capital of Qocho Kingdom (860–1138).
Badajoz (875 CE): Capital of Taifa of Badajoz (1010–1151).
Sort (c. 875 CE): Capital of County of Pallars (875–1011), County of Pallars Sobirà (1040–1201).

Around the same time and place

Reconquista (718–1492 CE): Centuries-long series of campaigns by Christian kingdoms to take the Iberian Peninsula from Muslim rule, ending with the fall of Granada in 1492.
Cyrillic script (c. 893 CE): Scholars in the First Bulgarian Empire, probably at Preslav, built a Slavic alphabet mainly from Greek letters; Cyrillic now writes Russian, Ukrainian, Bulgarian, Serbian and many more.
Trier (16 BCE – 1618 CE): Capital of Electorate of Trier (1333–1618).
Heraklion (c. 830 CE): Capital of Emirate of Crete (830–960).
Jaca (c. 830 CE): Capital of County of Aragon (830–921).
Treaty of Verdun (843 CE): Treaty dividing the Carolingian Empire among three grandsons of Charlemagne, roughly foreshadowing France, Germany and Italy.
Alfred the Great (849 CE – 899 CE): King of Wessex who defended his kingdom against the Vikings, reorganised its defences and promoted learning in English.
Palermo (c. 850 CE): Capital of Emirate of Sicily (850–1091), Kingdom of Sicily (1139–1266), Kingdom of the Two Sicilies (1815–1860).