Would a Roman army have dedicated engineers to design and build the siege engines (onagers, scorpions etc)? Would the engineers be expected to fight?
Personally, I am particularly interested in what was done around the time of the 2nd Punic war. But if someone provided an answer that described how things changed with military reforms, it would definitely be worth a bounty.
As the comment above indicates, the Roman army before Julius Caesar's time seems to have had a dedicated engineer corps, but this group would also be expected to fight if necessary. From Julius Caesar's rule onward, the Roman army retained a dedicated engineering officer or senior engineer called the Praefectus Fabrum, who could call upon specially trained or skilled legionaries to lead troops in construction tasks, which included the creation of roads, fortifications, siege engines and certain weapons.
Because Roman Army doctrine called for fortified camps to be constructed at the end of every day's march, which included defensive works, every legionnaire and auxiliary became, over time, an engineer by default. An interesting result of having most troops engage in building activities is discipline: busy troops, tired from marching and building, were less likely to mutiny, a constant threat in the politically-charged atmosphere in almost every Roman legion.
Yes, they had dedicated engineers for siege engines and works. Simple fortifications were handled by the legionnaires, but more complicated efforts had specialists.
No, not every army had them.
A specific example is Julian's army intended to attack Ctesiphon. In interest in speed, and because he did not believe Ctesiphon had walls, Julian did not take any engineers. Between the lack of any way to break the walls, and with the bulk of the Persian army marching their way, Julian was forced to abandon his attempt to sack Ctesiphon and attempt to get his army back to Roman territory.
