The Celts, also spelled Kelt, Latin Celta, plural Celtae were a collection of tribes which it is said originated in Central Europe but quickly expanded from the British Isles all the way to the Mediterranean. They even built a massive and vast roadway system long before the Roman Empire. In fact many of the Roman roads were built over ancient Celt roads. The Celts were not just known as fierce warriors but as amazing craftsman making the most intricate of jewelry, weapons and wares. It is believed they came into existence around 1200 BCE and became a true power throughout Europe controlling the regions of Northern Europe north of the Alps up to Great Britian and Ireland in the 3rd Century BCE.
Below is my favorite BBC documentary series that in my opinion covers the vast history of the Celts like none other.
At the bottom are several links as well for reading further on the Celts.
https://www.history.com/topics/ancient-history/celts