Cologne

City · c. 261 CE · in Europe

Capital of Gallic Empire (261–274).

Palmyra (c. 261–1929 CE): Capital of Palmyrene Empire (261–269).
Washim (c. 260 CE): Capital of Vakataka Kingdom (260–500).
Jeju (c. 263 CE): Capital of Tamna (263–1278).
Al-Hirah (c. 270–602 CE): Capital of Lakhmid Kingdom (270–602).
Kanchipuram (c. 283 CE): Capital of Late Pallava Empire (283–805).
Luoyang (c. 224 CE): Capital of Cao Wei (224–264), Western Jin (266–420), Eastern Wei (534–550), and 1 more.
San Marino (301 CE): Capital of Republic of San Marino (1797–1799).
Linfen (c. 306 CE): Capital of Former Zhao (306–329).
Ezhou (c. 207 CE): Capital of Eastern Wu (207–229).
Vijaya (c. 207 CE): Capital of Chámpa (207–1652).

Around the same time and place

Praetorian Guard (27 BCE – 312 CE): Augustus organised the Praetorian Guard in 27 BCE as the bodyguard of the Roman emperors. The guard came to make and unmake emperors, and Constantine the Great disbanded it in 312.
Trier (16 BCE – 1618 CE): Capital of Electorate of Trier (1333–1618).
Galen (c. 129 CE – 216 CE): Greek physician from Pergamon who practised in Rome; his medical writings dominated European and Islamic medicine for over a thousand years.
Amber Road (c. 1–400 CE): Baltic amber had been traded south since prehistoric times; in the Roman era the Amber Road ran from the Baltic coast via Carnuntum on the Danube to Aquileia on the Adriatic.
Diocletian (c. 244 CE – 311 CE): Roman emperor who ended the Third Century crisis, created the Tetrarchy and launched the last great persecution of Christians; he retired to his palace at Split.