Roman camp building

[Nathan Ross]

before the 10th legion (is it the 10th ?) crossed the Rubicon, did they construct a fortified camp ?

It was the 13th legion that crossed the Rubicon with Caesar.

I've often wondered about the fortified camps myself. Cisalpine Gaul (north of the Rubicon) was not part of Italy at this point; it was a province, which is why Caesar could legally keep his troops there. So would it be 'friendly territory' or not?

My guess would be that a regular camp would be established - you'd need somewhere to put c.5000 men, unless you were billeting them on the locals - and a camp would need a perimeter, tent lines etc. I suspect that the camp would not be fortified as strongly as it might be in enemy territory, but a commander would want to keep his men in practice!