If you are interested in this topic, you should consider reading Marc Morris’ “The Norman Conquest”, as it has a small paragraph related to this topic.

According to Morris, it was the Normans who brought castles to England. The clear evidence is the absence of castles before (they are hardly ever mentioned in English sources on warfare before the Conquest) and the number of castles the Normans built (because they couldn’t take over any existing ones). Bear in mind though, that these castles would not look like the Tower of London, but instead merely be wood-and-earth structures, maybe with a little bit of stone. These castles were cheaper to build than all-stone castles and worked well enough to quench Saxon rebellions.

The Normans needed castles because they were afraid of imminent Saxon revenge. There were quite a few rebellions after the Conquest and the Normans needed the castles for protection. Different reasons were at work on the continent. There, castles were the means by which ambitious nobles could press their claim on disputed territory. Essentially, castles were a sign of weak royal authority and geopolitical fragmentation into feuding factions. Normandy was one of these factions. So other than protection, the Normans just brought this tradition with them when they entered England.

So why didn’t the Saxons build castles before the Conquest? England, before the Conquest, wasn’t as fragmented as France. There certainly was a fair amount of strife, but the degree to which e.g. Normandy openly defied the French royal authority was unheard of and when there was a leadership crisis it didn’t result in fragmentation but in the substitution of the king. The Saxons did have fortifications, but these were communal defenses, burhs, not castles. The Anglo-Saxon nobility, the Thegns, lived in burhgeats, which were fortifications. But again, they played hardly a role during the Nordic invasions before the Conquest, so they probably weren’t very formidable and not comparable to the later castles.