But there were not wanting also walled towns, whose walls of alternate layers surprised the Romans both by their suitableness and by the elegant interweaving of timber and stones in their construction; while, it is true, even in the towns of the Allobroges the buildings were erected solely of wood.
Of such towns the Helvetii had twelve and the Suessiones an equal number; whereas at all events in the more northern districts, such as among the Nervii, while there were doubtless also towns, the population during war sought protection in the morasses and forests rather than behind their walls, and beyond the Thames the primitive defence of the wooden barricade altogether took the place of towns and was in war the only place of refuge for men and herds.
Intercourse In close association with the comparatively considerable development of urban life stands the activity of intercourse by land and by water.

Everywhere there were roads and bridges.
The river-navigation, which streams like the Rhone, Garonne, Loire, and Seine, of themselves invited, was considerable and lucrative.
But far more remarkable was the maritime navigation of the Celts.
Not only were the Celts, to all appearance, the nation that first regularly navigated the Atlantic ocean, but we find that the art of building and of managing vessels had attained among them a remarkable development.

The navigation of the peoples of the Mediterranean had, as may readily be conceived from the nature of the waters traversed by them, for a comparatively long period adhered to the oar; the war-vessels of the Phoenicians, Hellenes, and Romans were at all times oared galleys, in which the sail was applied only as an occasional aid to the oar; the trading vessels alone were in the epoch of developed ancient civilization "sailers" properly so called.( 14) On the other hand the Gauls doubtless employed in the Channel in Caesar's time, as for long afterwards, a species of portable leathern skiffs, which seem to have been in the main common oared boats, but on the west coast of Gaul the Santones, the Pictones, and above all the Veneti sailed in large though clumsily built ships, which were not impelled by oars but were provided with leathern sails and iron anchor-chains; and they employed these not only for their traffic with Britain, but also in naval combat.

Here therefore we not only meet for the first time with navigation in the open ocean, but we find that here the sailing vessel first fully took the place of the oared boat--an improvement, it is true, which the declining activity of the old world did not know how to turn to account, and the immeasurable results of which our own epoch of renewed culture is employed in gradually reaping.
Commerce Manufactures With this regular maritime intercourse between the British and Gallic coasts, the very close political connection between the inhabitants on both sides of the Channel is as easily explained as the flourishing of transmarine commerce and of fisheries.
It was the Celts of Brittany in particular, that brought the tin of the mines of Cornwall from England and carried it by the river and land routes of Gaul to Narbo and Massilia.

The statement, that in Caesar's time certain tribes at the mouth of the Rhine subsisted on fish and birds' eggs, may probably refer to the circumstance that marine fishing and the collection of the eggs of sea-birds were prosecuted there on an extensive scale.