At one time the Celts were as far ranging as Turkey and Spain.  They advanced out of Gaul with a dominate presence in the UK and pillaged Thrace and Macedonia, sacked Delphi and even sacked Rome itself.  From 500 BC to 43 AD they were the dominant European power.  Incredibly they never were a nation as we know it; they were simply a loosely unified group of individual tribes with a shared belief system and culture.

Like most European people the Celts were primarily farmers who lived in a loose town around their elected chiefs stronghold (oppidae).  The bonding force of this semi-nomadic people was the Druid class.  The Druids were priests charged with the duties of lawmaker, bard and wise men. 

Although the Celts traded with many other cultures, they were not heavily influenced by them having highly skilled artists and metalworkers of their own.  Their intricate designs were left on nearly everything they touched from jewelry and pottery to weapons.  When you can express your beliefs so thoroughly, there is little need for other forms.