The Dark Ages is a label applied to a time period where much of our knowledge about things was lost. If you've ever seen the amazing aquaduct system in Italy during the Roman Empire, you'll understand. That we tend to generalize--Lost? Really? No one else knew how to do that?--gives us a somewhat skewed view.
When the Normans landed in Pevensey on September 28th in 1066, they brought with them all that they needed to build a fort and had one up by nightfall! With a fort in place, the area could then be conquered and held. So I imagine the "average citizen" watching these guys with the strange helmets in awe as they hammered away at this monstracity they're building and suddenly it's up! To put this in perspective, I had to watch a whole season of This Old House to see their prefabricated Farm House put up! JK
Ordericus Vitalis later wrote there were "but few of the fortresses which the Normans call castles" in pre-Conquest England. King Harold had one built on the remains of a Roman fort. The Normans were bringing to England state of the art Continental technology and it was the Norman knight who had the knowledge.