The Celtic tribes of Europe picked up the idea of coins from Phillip of Macedon, who hired a good many Celtic mercenaries and paid them with Macedonian coins. Naturally, they copied Phillip's designs, rather crudely, and as time went on, each generation's copies became increasingly abstract - they literally dis-integrated - so that like an image copied and recopied on an old xerox machine, the original pattern eventually vanished into a swirling maze of seemingly random marks.

I have a book somewhere here about these old coins (Celtic Europe and Britain) that shows how this process worked. Like the Portrait of Dorian Grey.

Yes... that's all pretty much correct. Sometimes the "barbarisms" are the result of having a poor grasp of how to engrave the dies, but beyond a certain point its obviously deliberate. The Celt's strong point was abstract design -- convoluted swirls and transforming shapes, animals that are blossuming like flowers, letters that are intertwined with borders, that sort of thing.

I have a few other Celtic coins, but collecting them has become fashionable and too expensive for me. One of the coins I bought while I could still afford it was a quarter-stater from roughly the same period, also stuck by the Durotriges. There is something that is supposed to be a bristle-back boar on one side, and the "third geometic pattern" on the other. Evidently a purely abstract design. The only other Cletic British coin I have is a rough bronze from a slightly later era, called a Cunobelin AE Unit. It was struck by the Trinovantes while still technically "allies" of the Romans. (Later they rebelled along with the Icenii.) There's a flying Pegasus on one side, and supposedly Victory sacrificing a bull on the other. Other couple of Celtics I have are from Europe.

I've a bronze and a silver from Macedon, showing Alexander The Great's head. It's a little unclear to me whether either was minted in The Great's time though. Probably not. The type was struck for decades after his death. Wishful thinking by the Macedonians, who no longer controled the whole empire won by Alexander, and had't much future.