Built in Scotland between 1290 and 1300, Caerlaverock castle was one of the first castles of concentric circle design ever built. Caerlaverock is triangular in shape, of French style, with a large gatehouse of two drum towers standing at the apex of the triangle. At the time, these drum towers acted as the keep of the castle, and to bolster the defenses, a water-filled moat supplemented by an earthern embankment encircled the stronghold. Rounded towers were placed along the corners of the castle to provide flanking fire positions for the archers manning the towers. In 1300, Edward I siezed the castle, where it remained in English hands for the next twelve years before the constable of the castle defected to Robert the Bruce. Over the centuries, Caervalerock was repeatedly torn down and reconstructed.
J.E. Kaufmann, H.W Kaufmann, and Robert M. Jurga. The Medieval Fortress: Castles, Forts and Walled Cities of the Middle Ages. De Capo Press, 2001.
*image retrieved from http://www.castlexplorer.co.uk/