I've been reading an English translation of The Gallic War, and I have some questions about a few things.
Trenches and forts. Caesar's legions dig trenches and erect forts everywhere they go, and it doesn't seem to be a big deal at all.
For example at the Battle of the Aisne, Caesar orders one of his lieutenant-generals to "entrench a camp, with a rampart twelve feet high and a ditch eighteen feet broad". Later he constructs forts to protect his flanks, with the enemy Belgae only a kilometer or so away.
Where would the legions get the materials to build forts and ramparts so frequently, and how long would construction usually take? Would structures be dismantled after the battle (re-using materials)? And why would the enemy just watch while Caesar's men build forts so close to them?
Second: hostages. Caesar's takes a lot of hostages, what would their lives be like? Would they be put to work or treated more like guests? Would they even be kept with the army? Caesar takes 600 hostages from the Aedui, isn't this a bit excessive? I would have thought that having so many additional mouths to feed would cut in to the corn supply.
Theoretically, a Roman army is supposed to erect a "marching camp" every night, which basically consists of a rudimentary palisade and ditch. It is certainly not able to stop a determined foe, but the idea is to prevent an enemy from taking the Roman unawares, and given the rarity of succesful night raids against Roman armies this seems to have been effective, The material were simple wooden palisades, with the stakes carried by the soldiers themselves. Now, I say theoretical because I personally think this was actually very situation. Despite what manuals tell us, I doubt the commanders would spend a few hours of marching time building a palisade for no reason. The Romans were big on disciplinary exercises, so it isn't completely unreasonable, but if there was a need for speed and no enemy in the area I doubt they would have erected a fort. That being said, in Caesar's situation there certainly were nearby enemies, so forts would have been the norm.
It may seem like an unreasonable undertaking, and it certainly was impressive, but if you have five thousand men in a legion such a construction really wouldn't take that long.
For hostages, you hit on a big of a funny puzzle. You may notice that Caesar always takes hostages, and that it never works to actually control his enemies. It is frequently argued that this hostage strategy was simply ineffective against the Gauls, an only pursued for lack of other ideas. This will be thrown into really sharp relief when you get to Britain.
Anyway, the best accessible single volume source for the Roman military is Adrian Goldsworthy's The Roman Army at War. Worth a look if you like the topic.
http://www.reddit.com/r/history/comments/nmhpz/hi_all_2_questions_about_the_ancient_roman_era/
I got some good answers a couple of years ago.