How do we know all that was true, and not Roman propaganda?
True, there isn't any written, by them, evidence for Druidic practices; however, there are ancient Celitic stories, that though they have changed through the ages ( a la the game "TELEPHONE" ), regarding certain things.
Through ancient Roman writings, we DO know that the holy and oak trees were "sacred" to the Druids, AND even to this day, some aspects of Christmas and Halloween are STILL in practice today.
And then there's the newly found, OLDEST tales re Merlin, that are far different from the once oldest English and French versions.