Singidunum

The Scordisci — who may or may not have been ethnic Celts, but seem to have been part of a Celtic political constellation — had their capital at Singidunum (now Belgrade) in the 3rd century BCE, but the Scordisci faded from history more than 2000 years ago, after the Romans conquered them, and the rest of the Celts were pretty much gone from most of mainland Europe before the fall of Rome.

Discourse.net: Orthodox Celts 2009

Singidunum that they were not consulting their own interests in importing a leader from Thule against the wishes of the Emperor

Procopius History of the Wars, Books V. and VI. Procopius

Singidunum [82] and Sirmium, and extend as far as the Ister River.

Procopius History of the Wars, Books V. and VI. Procopius

Gepaedes got possession of the country about Singidunum [15] and

History of the Wars, Books III and IV (of 8) The Vandalic War Procopius

Next he invaded the city of Singidunum, which the Sarmatians themselves had seized, and did not return it to the Romans, but reduced it to his own sway.

The Origin and Deeds of the Goths Jordanes

Singidunum, town in the land of the Gepaides, modern Belgrade, III. ii.

History of the Wars, Books III and IV (of 8) The Vandalic War Procopius

Diocletian, when several suffered death for Christ at Singidunum

The Catholic Encyclopedia, Volume 13: Revelation-Stock 1840-1916 1913

The two bishops Ursacius of Singidunum and Valens of Mursia, who, on account of their Arianism, had been deposed by the Council of Sardica, now made a formal recantation of their error to Julius, who, having summoned them to an audience and received a signed confession of faith, restored to them their episcopal sees.

The Catholic Encyclopedia, Volume 8: Infamy-Lapparent 1840-1916 1913

The city of Belgrade was known until the seventh century A.D. by its Celtic name of Singidunum.

The Balkans A History of Bulgaria—Serbia—Greece—Rumania—Turkey Nevill Forbes 1906

Singidunum (Belgrade) in 471 and, after plundering Macedonia and Greece, settled in Novae (the modern Svishtov), on the lower Danube, in 483, where he remained till he transferred the sphere of his activities to Italy ten years later.

The Balkans A History of Bulgaria—Serbia—Greece—Rumania—Turkey Nevill Forbes 1906