ceiling; in these
cases the aisles only are vaulted, but in some small churches the
whole building has been so covered. Buttresses are seldom required,
owing to the great mass of the walls; when employed they have a very
slight projection, but the same strips or pilasters which are used in
German Romanesque occur here also. Low towers were common, and have
been not unfrequently preserved in cases where the rest of the
building has been removed. As the style advanced, the proportions of
arcades became more lofty, and shafts became more slender, decorative
arcades (Fig. 174) became more common, and in these and many other
changes the approaching transition to Gothic may be easily detected.

We have already alluded to the many Norman doorways remaining in
parish churches of which all other parts have been rebuilt. These
doorways are generally very rich; they possess a series of mouldings
sometimes springing from shafts, sometimes running not only round the
arched head, but also up the jambs of the opening; and each moulding
is richly carved, very often with a repetition of the same ornament on
each voussoir of the arch. Occasionally, but not frequently, large
portions of wall-surface are covered by a diaper; that is to say, an
ornament constantly repeated so as to produce a general sense of
enrichment.

[Illustration: FIG. 173.--NAVE ARCADE, PETERBOROUGH CATHEDRAL.]

[Illustration: FIG. 174.--DECORATIVE ARCADE FROM CANTERBURY
CATHEDRAL.]

Norman castles, as well as churches, were built in great numbers
shortly after the Conquest, and not a few remain. The stronghold
which a follower of the Conqueror built in order to establish himself
on the lands granted him was always a very sturdy massive square
tower, low in proportion to its width, built very strongly, and with
every provision for sustaining an attack or even a siege. Such a tower
is called "a keep;" and in many famous castles, as for example the
Tower of London, the keep forms the nucleus round which buildings and
courtyards of later date have clustered. In some few instances,
however, as for example at Colchester, the keep is the only part now
standing, and it is probable that when originally built these Norman
castles were not much encumbered with out-buildings. Rochester Castle
is a fine example of a Norman keep, though it has suffered much from
decay and injury. The very large Norman keep of the Tower of London,
known as the White Tower, and contai