t a prolonged imprisonment.

The Tower of London as it exists to-day has, by a slow process of
gradual accretion round the keep as a nucleus, become what is known as a
"concentric" castle, or one upon the concentric plan, from the way in
which one ward encloses another; and its architectural history falls,
roughly speaking, into three chief periods covered by the reigns of
William Rufus, Richard I., and Henry III., all the more important
additions to the fortress occurring approximately within these periods,
as will be seen later on.

Commencing with the building of the great keep (now called the White
Tower), and the small inner or palace ward to the south of it, by
William the Conqueror, this at first was probably only enclosed by a
stout timber palisade on the top of a raised bank of earth, having a
ditch at its base. The first recorded _stone_ wall round the Tower was
that of William Rufus, already mentioned, and it is not improbable that
the wall marked "v" on the plan (only discovered in 1899 during the
erection of the new guard house) may have formed part of his work.

But little is known to have been added by Henry I. The sole remaining
Pipe Roll of his reign only records a payment of L17 0s. 6d. "in
operatione Turris Lundoniae," without any further mention of what these
works were, and as the amount is not very large, it is not probable that
they included anything of much importance. That the smaller inner or
palace ward to the south of the keep was already completed, is shown by
a charter of the Empress Maud, dated Midsummer, 1141, which granted to
Geoffrey de Mandeville (then Constable of the Tower, and third of his
family to hold that important office) the custody of the Tower, worded
as follows: "Concedo illi, et heredibus suis, Turris Lundoniae cum
'parvo castello' quod fuit Ravengeri";[22] and this "little castle" is
the before mentioned inner or palace ward, though how or where this was
originally entered from the city nothing now remains to tell us--most
probably at or near the point subsequently occupied by the Cold Harbour
Gate "u," at the south-west angle of the "turris," or White Tower
"r," for it is but seldom that the original entrance gates of castle
baileys or courtyards are removed, unless in the case of an entire
re-arrangement of the plan, with the consequent rebuilding thereby
rendered necessary.

Owing to the state of anarchy that prevailed during the troubled reign
of Stephen, and the