Arrival of Celts to the Celje basin was more than merely displacement of one tribe by another. The Celts were a widespread ethnical group with a strong state organization characteristic of the entire Central and Western Europe at the time. They introduced the Latenic culture notable especially for iron objects. In the centuries BCE, Keleia became one of the most important settlements (oppidum) in the Celtic Noricum, maintaining economic connections with the Roman predecessors. Especially the Noric iron, as well as gold, salt, and other merchandise that was always in strong demand in the wealthy Rome, improved the reputation of the town on the Savinja River. It is therefore hardly surprising that Keleia, becoming one of Noricum's most important craft and trade hubs in the 1st and 2nd century BCE, which even operated its own mint of large and small silver coins, was known even to the Ancient Greeks.
The favourable location of this Celtic settlement on the Savinja River, then navigable, and on the amber road that crossed this region, connecting the Baltic and the Mediterranean, was appreciated early on by the Romans who annexed the Noric Kingdom (Regnum Noricum) in the year 16 BCE, without a fight.
In addition to the rich material culture, the Celts left their successors numerous names for places and rivers. Not only the name Celje (Keleia) – Savinja and Sava, too, are of Celtic origin, just like the names of most mountains in the eastern Alpine forelands.