It is to be noted that, although the architecture of the castle and the fortifications of towns naturally took its share in the formal progress of Gothic art, the laws under which it was evolved bear no resemblance to the principles of construction, in obedience to which the medieval cathedral assumed its characteristic form. Ribbed vaults, Gothic mouldings, and traceried windows afford a clue to the dates of the various parts of a medieval castle, as they do to those of a church; but they are merely incidental to a type of construction to which the solid and impregnable wall is all-important. The cases are rare in which the builders of castles paid much attention to elaborate detail in the minor parts of their building: their decorative work is used with the economy and simplicity appropriate to the massive construction which their fortresses demanded. A vast amount of work still remains to be done in the exploration of our military buildings and the reconstruction of their history; and, until that is accomplished, no thoroughly satisfactory general hand-book can be written. Nevertheless, it is hoped that there is room for books which may serve as general indicators to what has been done, up to the present time, in this direction. The bibliography which will be found preceding the text of this volume includes a selected list of monographs or articles upon individual castles, many of which have appeared in the transactions of various archaeological societies. These vary considerably in value; but, taken as a whole, they serve to enlarge our knowledge of the history and architecture of the buildings with which they are concerned.