Henry VIII became concerned with the threat of French invasion during 1539 and was familiar with the more modern continental designs. He responded to the threat by building a famous sequence of forts, called the Device Forts or Henrician Castles, along the south coast of England specifically designed to be equipped with, and to defend against, gunpowder artillery. These forts still lacked some of the more modern continental features, such as angled bastions. Each fort had a slightly different design, but as a group they shared common features, with the fortification formed around a number of compact lobes, often in a quatrefoil or trefoil shape, designed to give the guns a 360-degree angle of fire. The forts were usually tiered to allow the guns to fire over one another and had features such as vents to disperse the gunpowder smoke. It is probable that many of the forts were also originally protected by earth bulwarks, although these have not survived. The resulting forts have been described by historian Christopher Duffy as having "an air at once sturdy and festive, rather like a squashed wedding cake".