Did Romans build fortified castles to protect vital areas of the Empire?
It depends on your definition of a castle, as Romans built thousands of forts throughout their Empire in the centuries in which they were at their height. Most towns and cities which have Roman origins originally began as fortified settlements for either soldiers or Roman settlers, which would be well defended against any form of possible attack. The concept of a castle, however, is largely reserved for the Medieval period, although the word castle itself comes from the Latin “castellum”, which means fort. So, in short, yes technically Romans built castles throughout their empire, but not in the style or design as was prevalent in the Medieval period.