Cæsar had no knowledge of such a bridge, or at least did not march
towards it, but crossed the river with difficulty in the higher
reaches by a ford. And it may also be urged that a bridge across the
Rhine was equally unknown in that time. But, the bridge once
established, it could not fail to become the main point of convergence
for the commerce of Southern England, and indeed for much of that
which proceeded from the North upon its way to the Continent. Such an
obstacle would oppose itself to every invasion, and did, in fact,
oppose itself to more than one historical invasion from the North Sea.
It would further prevent sea-going vessels whose masts were securely
stepped and could not lower from proceeding farther up stream, and
would thereupon become the boundary of the seaport of the Thames. Such
a bridge would, again, concentrate upon itself the traffic of all that
important and formerly wealthy part of the island which bulges out to
the east between the estuary of the Thames and the Wash, and which
must necessarily have desired communication both with the still
wealthier southern portion and with the Continent. But, more important
than this, London Bridge also concentrated upon itself all the
up-country traffic in men and in goods which came in by the natural
gate of the country at the Straits of Dover, except that small portion
which happened to be proceeding to the south-west of England: and this
exception to the early commerce of England was the smaller from the
comparative ease with which the Channel could be crossed between
Brittany and Cornwall.

Finally, the Bridge, as it formed the limit for sea-going vessels,
formed also if not the limit at least a convenient terminus for craft
coming from inland down the stream. It would form the place of
transhipment between the sea-going and the inland trade.