The romans built aqueducts, And castles where avaible during this era aswhell. Well, It was walls but some of them looked like castels. espeshally the big wals around som citys in the middle east. those stone walls where werry big and had the same effect as the castles. to get castels avible at masonary is early, construction sounds good but maths sounds greater imo.
Dragon,
I agree that the Romans had defensive works, many of which were impressive. They did differ from castles, though. It is similar to the points above, fortification developed over time and it is sometimes hard to say when large military advances occurred.
Clearly, though, castles were largely a medieval invention and the offense/defense equation was different in the medieval period than in the Roman period.
Medieval castles were really expensive. They were made as permanent structures for a SMALL group of people to show defensive power. A noble would establish a town, give out a fief, and build a castle. When a raid occurred, the townsmen moved into the castle and defended it. It is important to remember how small Medieval armies were, and the importance of castles in a social context.
The Romans would rarely spend the kind of money that say Edward I did in castles, largely because their armies and populations were quite large. They needed larger scale defensive measures. So, for instance, Hadrian built a large war in northern England, not a series of castles (which is what a medieval ruler like Edward would have done). Conversely, the Normans brought castles to England, and it helped the small Norman leadership to control a large, initially hostile population.
Medieval castles were also more sophisticated than Roman forts, but the biggest difference is the way that combat occurred as a whole. Even if the Romans had the technology for castles, I'm not sure they would have used them. And, as you said, they did have quite effective fortifications. But the Romans didn't use the 'usual' features of castles, like battlements, crenellations, portculli very often.
Best wishes,
Breunor