To repeat myself for the second time, this time for you Cato, the quote that prompted me to ask the question, was the below from Goldsworthy:-
"The defences of most Roman forts would have posed few problems for an army with some knowledge of siegecraft. However, for much of the Principate only the Romans possessed this technology."
It doesn't say they invented the technology, nor that people prior to the Roman supremacy didn't possess it, as in the case of Alexander. It merely states that for "much of the Principate only the Romans possessed this technology".
If you disagree, fine, state your case. Give some examples, from the first say, 200 years of the Principate, where we can see another state using digging works, circumvellation, siegetowers, sappers etc.
With regard to Dacia, I am sure they could counter-tunnel if under siege, as the Jews did at Jerusalem, but would love to see that source you mention Princeps.
We all have opinions, till now no ones posted any proof. I'd love so find out something to the contrary.
For absolutely the last time - I am talking 50BC to say 200AD and at present I agree with Goldsworthy in saying that the Romans faced so serious threat of siege, as only they had the technology DURING THAT PERIOD.