Although France has been described as "the heartland of medieval architecture", the English were at the forefront of castle architecture in the 12th century. French historian François Gebelin wrote: "The great revival in military architecture was led, as one would naturally expect, by the powerful kings and princes of the time; by the sons of William the Conqueror and their descendants, the Plantagenets, when they became dukes of Normandy. These were the men who built all the most typical twelfth-century fortified castles remaining to-day". Despite this, by the beginning of the 15th century, the rate of castle construction in England and Wales went into decline. The new castles were generally of a lighter build than earlier structures and presented few innovations, although strong sites were still created such as that of Raglan in Wales. At the same time, French castle architecture came to the fore and led the way in the field of medieval fortifications. Across Europe – particularly the Baltic, Germany, and Scotland – castles were built well into the 16th century.