Celticisation, or Celticization, was historically the process of conquering and assimilating by the ancient Celts. Today, as the Celtic inhabited-areas significantly differ, the term still refers to making something Celtic, usually focusing around the Celtic nations and their languages.
During the first millennium BC, the early Celts expanded from a core territory in Atlantic Europe to Iberia, the British Isles and later also the Balkans and central Europe, and are assumed to have "Celticized" (Pre-Celtic) earlier populations such as Illyrians and Thracians in the Balkans and Basques elsewhere.
The Celticization in Pannonia began as early as the 4th century BC.La Tene type finds are characteristic in Pre-Roman Pannonia and are considered a marker to variations in the degree of Celticization. Among the Illyrian tribes some were Celticized to varying degrees(some completely) like the Pannoni and the Dalmatae. A type of wooden oblong shield with an iron boss was introduced to Illyria from the Celts. Illyrian chiefs and kings wore bronze torques around their necks much like the Celts did.
The Celts had two settlements that later became cities in Illyria, namely Navissos and Segestica. In Thrace they had Serdica (modern Sofia, Bulgaria),Tylis, founded by Gauls, Dunonia, Singidunum and Taurunum.