The Origins of Castles

When do we first see castles in Britain? What's the first castle that we know of? I go with the traditional argument, which is that the Norman conquest is the point when they first come into England. It's a little bit more nuanced than that, in that there are a handful of castles that predate the conquest by 15, 20 years. The origins of castles are sort of you know, a bit like the migration of ewels. They're quite novel in Western Europe as a whole.