In 298 BCE, Celtic tribes reached what is today Bulgaria and clashed with the forces of Macedonian king Cassander in Mount Haemos (Stara Planina). The Macedonians won the battle, but this did not stop the Celtic advancement. Many Thracian communities, weakened by the Macedonian occupation, fell under Celtic dominance.^[12]

In 279 BCE, one of the Celtic armies, led by Comontorius, attacked Thrace and succeeded in conquering it. Comontorius established the kingdom of Tylis in what is now eastern Bulgaria.^[13] The modern-day village of Tulovo bears the name of this relatively short-lived kingdom. Cultural interactions between Thracians and Celts are evidenced by several items containing elements of both cultures, such as the chariot of Mezek and almost certainly the Gundestrup cauldron.^[14]

Tylis lasted until 212 BCE, when the Thracians managed to regain their dominant position in the region and disbanded it.^[15] Small bands of Celts survived in Western Bulgaria. One such tribe were the serdi, from which Serdica - the ancient name of Sofia - originates.^[16] Even though the Celts remained in the Balkans for more than a century, their influence on the peninsula was modest.^[13] By the end of the 3rd century, a new threat appeared for the people of the Thracian region in the shape of the Roman Empire.