Why were Celtic Oppida’s abandoned?

[Tim Hare]

My guess is trade, and the lack of it. At Caerwent, Wales, the legion fort had trade through the new Roman road and the Troggy Brook, actually a small river in boats could travel up it. The difference between a river and a brook is, you can pull a loaded boat through it. (There were some shockingly narrow rivers through the dales of England.) There was also the small trade of supplying a legion. The hill fort a couple of miles away was abandoned. It's hard to trudge your trade goods up the hill for market day, when you can take a boat to market.

The death of the Celtic steel industry in 'Germany' came when Ceaser destroyed the trade partners in Gaul.
Hillforts are too far from the river fords, where the Romans put their forts. Trade happens at river fords. London, Frankfurt and any other big city on a river.

Thanks for taking the time to put your thoughts across.

Question in regards to your theory on Celtic steel industry. Why would Roman occupation of Gaul destroy this industry in Germany?

Surely the Romans would simply replace the Gauls as potential trade partners, accept with more gold and silver to trade and much larger consumer base to feed. Typically a wealthy neighbour increases opportunities for trade.

In regards to Oppida’s as trading centres, surely ‘competition’ from the better situated Roman forts would only impact those geographically close to the frontier. Would it really cause Oppida’s hundreds of miles away to go out of business economically? (That would imply the vast majority of Germans who might otherwise travel to an Oppida to trade for goods or pay for services found themselves within range of a Roman fort, which I can’t imagine would be the most inviting of places even in peace times)