The following cathedrals also were built in the Norman period, and still retain portions of the original work:--Lincoln, Rochester, Ely, Worcester, Gloucester, Durham, Norwich, Winchester, Peterborough, and Oxford. Castles were erected in various parts of the kingdom, to restrain the rebellious people, who could ill brook the tyranny of the Conqueror. Of these the Tower of London is one of the most important, and the chapel in the White Tower is one of the best examples (dated 1081) we possess of early Norman, though, from its situation in a military building, it has less of ornament than might otherwise have been expected. Of Norman castles, the chief parts which remain are the keeps or principal towers, and these have ordinarily one prevailing character. They are square masses, not having much height in proportion to their breadth, and merely relieved at the angles by slightly projecting turrets. The windows are in general comparatively small, and the walls exceedingly thick--sometimes, as at Carlisle, reaching to sixteen feet. Norwich, from its immense size, is an excellent sample of this kind of tower, and Castle Hedingham is another. Of the houses of this period many yet exist, though not in an entire state; and of these some fine specimens are found in Lincoln,[35] where they are said to have belonged to the Jews, but whose riches at that time only led to their destruction. Many rich and magnificent examples of monastic buildings of this date occur in various parts of the kingdom. Norman architecture may be divided into three periods--namely, Early, Middle, or fully-developed, and Transition; the first extending from the Conquest, or a few years previous, to the end of the reign of Henry I., 1135; the second from the commencement of Stephen to nearly the end of Henry II., 1180; after which date the Transition commences, and the style gradually loses its characteristics until it merges in the succeeding, or Early English style of the thirteenth century. Of the first period, the chapel in the Tower of London has been already mentioned as an example; the second includes most of our rich Norman buildings; and of the third, the Temple Church is a good specimen. The great characteristics of Norman architecture are solidity and strength. Walls of an enormous thickness, huge masses of masonry for piers, windows comparatively small, and a profusion of peculiar ornament, seem to be essential to the full development of the style; and there is a gloomy magnificence in a fine Norman building which is highly impressive; its walls seem as firmly fixed in the earth as the iron foot of the Conqueror was on the neck of a prostrate nation. The distinctive features of Norman architecture may be thus summarised:--