Large fortified building.
Had its heyday in the pre-gunpowder era.
Techniques originally created in classical times and perfected by the Romans. Skills to build such structures were lost in the west during the Dark Ages but slowly rediscovered from about 1000ad onwards with Moat and Bailey castles. Reached perfection with huge structures like Caernarvon Castle, The Tower of London or Beaumaris Castle when cannon had made such structures obsolete.