Although they did not ultimately prevaill over the Romans, the Celts really should be appreciated more on a military level. Ultimately, lots of smaller confederations or individual tribes did incredibly well against what was effectively a nation state in a way almost unheard of outside of nomadic civilisation.
Certainly their use of psychological warfare in a culturally consistent way is incredible. Wherever the romans fought them, instigating fear and in often linked way across multiple countries was central to the celtic tactic. Even the fall of the druids was noted, in common fashion, as terrifying to the Romans even in their victory.
"On the shore stood the opposing army with its dense array of armed warriors, while between the ranks dashed women, in black attire like the Furies, with hair dishevelled, waving brands. All around, the Druids, lifting up their hands to heaven, and pouring forth dreadful imprecations, scared our soldiers by the unfamiliar sight, so that, as if their limbs were paralysed, they stood motionless, and exposed to wounds." - Tacitus.