I would not say they were that primitive.
They had written word and literature, iron and copper mines, gold for currency, a triad of spiritual leaders, a well developed agriculture and a ruling warrior class with almost a feudal system. They had laws and trade, schools and guilds of tradesmen. They traveled allover Europe, North Africa and the Near East. They Buried their Dead cremated in urns, ritual sacrifice did happen but it was an organised religious activity.
Their Kings ruled for seven years, and a contract of marriage lasted a year and a day. Rome considered themselves more civilised, but in many ways not so.
GothicNature, Harkon is correct. The Celts actually saw the Romans as uncivilized and rather quite a dirty and wasteful people, so it can easily be counted as a two way street with viewpoints on the civilizations we're discussing.
A good example of the cultural differences would be "blood pudding", something brought to Britain by the Romans. Before the Roman influence, when an animal was slaughtered (either ritually...and yes, I concede that was done... Or for feeding the tribe) the blood was used to fertilize the fields, a common practice these days as you can go down to the local Nursery and buy blood-meal fretilizer.
The Celts were also using soap while the Romans were still oiling themselves up and scraping it off their skin with sticks after attending the communal baths.
The Brehon Laws (early Irish laws handling everything from theft, to murder, and even divorce) are still studied at law schools around the world. The Brehons (a title) themselves were actually an early from of travelling magistrate.
Merely different cultures, dear lady, certainly no less civilized than the other.
How about this part:
Other than Caesar's account in De Bello Gallico (his own writings about the events of the Gallic wars that he started), I do not know of any other referrences to such as this.
Again, Caesar was a politician trying to undermine the public standing of the Celts and their Druids.
As to the Druids making up rituals as they went along, well, I'd say possibly. But, we will never be albe to tell for certain (just like the human sacrifice speculation) as there are no Ancient Druids around to ask. Druid traditions and practices were only passed along by "lip to ear", in other words an oral tradition. Some of this was written down by monks, centuries later, and a bit of Christian "spin" put on some points that may have seemed objectionable in the fourteenth century and later. The same was done with the tales of King Arthur, who was originally known as a "War Leader" not as a King. Guenivere's "unfaithfulness" was another bit of the tale changed for a more Christian world view.
So, until we find some long lost written account of what the Ancient Druids did, believed, and thought, (or a living two thousand year old Druid) we have to separate the wheat from the chaff...and Caesar's account is just that, chaff.
There is a graphic illustration of Divining by Blood Sacrifice in the Graphic Novel "Slaine the King" (First published by 2000AD).
A cowardly King is sacrificed so the future can be seen by the flow of his blood from the stone. The trickle of life fluid is seen to pool at the feet of the warrior Slaine, indicating that he must trial as the next Horned God figure, the King of the Land of the Young.
These novels might not be Historically accurate, but they show in detail the mythology and Belief Practice of the Celtic People.
I'm sorry, I just cannot accept that as any more historically accurate to Celtic beliefs and practices than I can Robert E. Howard's Conan series.
We only have second hand information (via Caesar) that there were sacrifices as such. Such later scribed tomes as The Yellow Book of Lecan, The Red Book of Hergest, and the Black Book of Carmarthen (some of the primary sources for Celtic Reconstructionists) don't hold any accounts of divinatory sacrifice like this, to the best of my knowledge. Even if they did, it would certainly be immoral and impractical to conduct such things in this day and age so it becomes a moot point, as those following a Druid Path in the Twenty First Century wouldn't even consider such a thing.
We do not have an unbroken and untainted line of information as to what the Ancient Druids did or believed. We have accounts that were written down centuries after, accounts from people that did not understand the different society from their own that they were observing, archaeological evidence showing what "might" have been, and some surviving local customs that can be related to the early Celts. We have nothing that directly shows us the thoughts and beliefs of these ancient peoples. We have no first hand account, such as a personal diary, of these peoples. I will honestly say that I wish we did!
Even the term "Celt" (again, stemming from Caesar's writings about a tribe in Gaul called the Keltoi) which used to be accepted as a specific race of people has been shown to represent a culture rather than genetics.
Druids and Jesus Christ Discussion: Did Jesus Meet Druids At Glastonbury?