SOMETIMES, walking around this thought-provoking exhibition, you can’t helped wondering if "Celts" wasn’t just a word used in a mildly pejorative way by the Ancient Greeks for any old Barbarian, of a particular bent, worrying at the fringes of their civilization. Certainly the Greeks tarred most of the Northern tribes surrounding their borders with the same ‘Celtic’ brush, and the Romans did too – "Gauls" were interchangeable with "Celts", although interestingly enough, the Romans didn’t call any of the tribes of Britain, Celts.

Confused? You will be. If the Celtic languages – Welsh, Scottish and Irish Gaelic, Breton, Manx and Cornish – developed from one European root (and arguably their culture too), the rest of the twirly, ornate interlaced history of the Celts is a modern concoction, suggests this exhibition.

And it is, indeed, complicated stuff. The movement of tribes and peoples across Europe from the Iron Age onwards is one which is hard to ascertain with any certainty. The phenomenal Gundestrup Cauldron, on loan from the National Museum of Denmark, is testament to a mingling of cultures, its diverse and idiosyncratic imagery the subject of many an academic paper, its origins as obscure as the cultural influences on its fabulous and mysterious hammered silver sides.

Organised jointly with the British Museum and shown in slightly reduced form here, after its initial showing in London in the Autumn, Celts is an exhibition that staggers under the weight of sheer artistic brilliance. In the end, you may not care whether the Celts are a homogenous tribal group or a 19th century fabrication, the artefacts here are a jaw-dropping testament to sophisticated and alien interlinked cultures stretching as far back as 500 BC.

Everywhere there are unanswered or partially-answered questions, from outlandish helmets, with huge conical horns (or elsewhere, unwieldy animalistic accoutrements) that might seem more designed as status symbols than for any use in battle, to the incredibly weighty jewellery that gleams from the display cases. Fantastical creatures and hidden faces abound, eyes look out from unexpected angles, finely wrought vegetation tangles and trails to greater or lesser extent depending on the period. Art was used to impress or to terrify, to tell stories, to signify, or simply to decorate, as indispensable on a war helmet as on a tiny clothes pin. This is the stuff that archaeologists, metal detectors and mudlarkers dream of when they pack away their trowels for the night.

At the far end of the gallery, a series of vitrines full of torcs – ornate neckpieces – is a smorgasbord of artistic and metallic might, a display of torc one-up-manship that staggers, from the crown-like heft of the piece found in Rodmose (Jutland, Denmark, 250-100 BC) to the finely wrought double-helix of the Blair Drummond torcs (Stirling, 300-100 BC). Even more fascinating, perhaps, are the finds from Snettisham, Norfolk, a presumed ceremonial site discovered when a farmer engaged in some heavy plowing in the 1940s, and since revealing a staggering wealth of Iron Age jewellery and coinage. The tiny photo of a hole in the ground filled with gold torcs is thrilling and thought-provoking in equal measure.

Elsewhere, fantastical beasts and alien imagery abound, not least the boar-headed carynx – essentially a ceremonial war trumpet, according to experts including John Kenny, the trombonist and carnyx player, and one shared with cultures beyond those recognized as Celtic. The only carnyx found on British soil – a burial offering – found at Deskford in Aberdeenshire is shown alongside French examples from Tintignac, with its wild and large ears that would have acted as a "loudspeaker" for the unearthly and multifaceted sounds produced by the instrument itself. The look is, indeed, as terrifying as the sound – the accompanying videos on the Deskford reconstruction and playing are hugely insightful – although here, as elsewhere, it is perhaps the alienness of the thing that gives us a reaction that our forebears, Celtic or otherwise, would not necessarily have recognized.

But the less outlandish items startle and amaze too, from a miniature bird-topped pestle, designed for crushing mineral pigment for self-adornment, to a small bronze boar, designed, perhaps, as a helmet topper (one wouldn’t be surprised), from a time when boar were far more wild than domestic. A carved comb too, from the period just before the Roman invasion of Britain, suggests that the indigenous hirsute barbarians perhaps felt the need to tidy up in the face of the first trickle of clean-shaven Romans. The exhibition moves gradually from the Iron age through to the mediaeval, via religious sculpture and manuscript illustration, to the renewed interest in the Celts in the 1700s.

If this exhibition’s aim is to suggest that our modern idea of the Celts as a homogenous tribe is incorrect, that our Celtic image is as layered and reworked as the multiple panels on the Gundestrup Cauldron, it does so by conjuring a wild if nebulous image of a Celtic culture itself, spanning the millennia. What it does, too, is demonstrate how innovative and sophisticated artistic endeavour was long before the "civilizing" Romans. The craftsmanship and artistry in the Braganza Brooch, from the Iberian peninsula c. 250 BC, is a staggering reprimand to anyone who thinks that intricate workmanship and sophisticated storytelling are the preserve of more modern cultures.

Celts is at the National Museum of Scotland, Chambers Street, Edinburgh until September 25