Although Britain presents numerous examples of military works, and her Welsh and Scottish borders are very thickly set with castles, the circumstances of the country have not been favourable to the production of military buildings of the first class. Our insular position has enabled us to dispense altogether with grand frontier fortresses; and our great nobles, although they often held their own against the Crown, and even encroached upon its legitimate powers, drew their resources from estates more or less scattered in position, and seldom possessed whole provinces, or ruled over a territory sufficiently compact and extensive to justify the construction of a great castle-palace like those of France, for the defence of the lordship and the residence of the baron. The keeps of Arques, Etampes, Provins, and Vez; the towers of Coucy and Beaucaire; the walls of Avignon; and the fortresses of Château-Gaillard, Carcassonne, Villeneuve-les-Avignon, and Pierrefonds, the details of which are familiar to the readers of the exhaustive work of M. Viollet-le-Duc, are due to a period when France was divided into provinces, the rulers of which were scarcely subordinate to its Crown, and were either actual monarchs elsewhere, or held much of the state privilege and power of independent sovereigns. It happens, however, that, in that particular class of fortress of which the quadrangular Norman keep is the type, we have less to fear comparison, seeing that castles of this description are confined, or very nearly so, to our own country and to Normandy. Whereas, on the continent of Europe, in Italy, Spain, France, and Germany, the earlier castles appear to have sprung directly from Roman, or debased Roman patterns, in Normandy a simpler and more original type prevailed, unlike what is seen in other parts of France, and which there is some reason to regard as the invention of the Normans themselves. These keeps, so remarkable for their simple quadrangular form and the immense solidity of their masonry, were erected in Normandy during the eleventh century, and are well known by such examples as Arques near Dieppe, Falaise, and Caen. By the Normans they were introduced into England.