As against the _castellum_, the fortified enclosure, we have a new and distinct type of fortress, the outcome of a different state of society, in the single "motte" or "tour." I shall not here enter into the controversy as to the relation between these two forms, my space being too limited. For the present, we need only consider the "motte" (_mota_) as a mound (_agger_) crowned by a stronghold (whether of timber or masonry), but _not_, as Mr. Clark has clearly shown, "crowned with the square donjon," as so strangely imagined by Mr. Freeman.[963] In the "tour" (_turris_) we have, of course, the familiar keep of masonry, rectangular in form, and independent of a mound. The process, then, that we are about to trace is that by which the "motte" or "tour" coalesced with the _castellum_, and by which, from this combination, there was evolved the later "castle." For my theory amounts to this: in the mediæval fortress, the keep and the _castellum_ were elements different in origin, and, for a time, looked upon as distinct. It was impossible that the compound fortress, the result of their combination, should long retain a compound name: there must be one name for the entire fortress, either "tour" (_turris_) or "chastel" (_castellum_). Which was to prevail? This question may have been decided by either of two considerations. On the one hand, the relative importance of the two factors in the fortress may have determined the ultimate form of its style; on the other—and this, perhaps, is the more probable explanation—the older of the two factors may have given its name to the whole. For sometimes the keep was added to the "castle," and sometimes the "castle" to the keep. The former development is the more familiar, and three striking instances in point will occur below. For the present I will only quote a passage from Robert de Torigny, to whom we are specially indebted for evidence on military architecture:— (1123) "Henricus rex ... turrem nihilominus excelsam fecit in castello Cadomensi, et murum ipsius castelli, quem pater suus fecerat, in altum crevit.... Item castellum quod vocatur Archas, turre et mœnibus mirabiliter firmavit.... Turrem Vernonis similiter fecit."[964] More interesting for us is the other case, that in which the "castle" was added to the keep, because it is that of the respective strongholds in the capitals of Normandy and of England. The "Tower of Rouen" and the "Tower of London"—for such were their well-known names—were both older than their surrounding wards (_castra_ or _castella_). William Rufus built a wall "circa turrim Londoniæ" (_Henry of Huntingdon_):[965] his brother and successor built a wall "circa turrim Rothomagi."[966] The former enclosed what is now known as "the Inner Ward" of the Tower,[967] the "parvum castellum" of Maud's charter.[968]