elevation for a
stronghold. What military reasoning led William of Falaise to discern
it at once and there to build his keep?

In order to answer that question let us consider what other points in
the valley were at his disposal.

Reading we have discussed. The chalk spurs in the gorge by Goring and
Pangbourne are not isolated (as is that of Chateau Gaillard, for
instance), and are dominated by the neighbouring heights. The
escarpment opposite Henley offered a good site for an eleventh-century
castle--but the steep cliff of Windsor had this advantage beyond all
the others--that it was at exactly the right distance from London.
Windsor is the warden of the capital.

If the reader will look at a modern geological map, he will see from
Wallingford to Bray a great belt of chalk in which the trench of the
Thames is carved. Alluvials and gravels naturally flank the stream,
but chalk is the ground rock of the whole. To the west and to the east
of this belt he will notice two curious isolated patches, detached
from the main body of the chalk. That to the west forms the twin
height of the Sinodun Hills, rising abruptly out of the green sand;
that to the east is the knoll of Windsor, rising abruptly out of the
thick and damp clay. It is a singular and unique patch, almost exactly
round, and as a result of some process at which geology can hardly
guess the circle is bisected by the river. If ever the chalk of the
north bank rose high it has, in some manner, been worn down. That on
the south bank remains in a steep cliff with which everyone who uses
the river is familiar. It was the summit of this chalk hill piercing
through the clays that the Conqueror noted for his purpose, and he
was, to repeat, determined (we must presume) by the distance from
London.

The command of a great town, especially a metropolis, is but partially
effected by a fortress situated within its limits. In case of a
popular revolt, and still more in case the resources of the town are
held by an enemy, such a fortress will be penned in and find itself
suffering a siege far more rigorous than any that could be laid in an
open country-side. On this account the urban fortresses of the Middle
Ages are to be found (at least in large cities) lying upon an extreme
edge of the walls and reposing, as far as possible, upon uninhabited
land or upon water, or both. The two classic examples of this rule
are, of course, the Tower and the Louvre, each standing down stre