Bretons in France can be proud - they were the only who survived in continental Europe. The continental subgroup of Celtic languages is now extinct, Gaulish , Lepontic and Celtiberian languages died out, that is why we can judge about Common Celtic only by the remains of Continental subgroup and by living Insular subgroup languages. Common Celtic shows no initial mutations, lenition or nasalisation etc. Gaulish did not have them either. This is the feature that was developed in Insular Celtic epoch, though arguments are still under way.
Some noun endings show archaic instrumental case in dative constructions.
And practically nothing is known for sure about the verbs in Common Celtic.