The fact is that the Celts of the southern seaports were the only ones who understood shipbuilding, which they had learnt from the Phoenicians, and the only ones who were civilized enough to unite among themselves and with their fellow-Celts in what now is France but then was Gaul. The rest were mere tribesmen under chiefs who were often squabbling with one another, and who never formed anything like an all-Celtic army. For most of them a navy was out of the question, as they only used the light, open-work, basket-like coracles covered with skins--about as useful for fighting the Romans at sea as bark canoes would be against real men-of-war. The Roman conquest of Britain was therefore made by the army, each conqueror, from Caesar on, winning battles farther and farther north, until a fortified Roman wall was built across the narrow neck of land between the Forth and Clyde. Along these thirty-six miles the Romans kept guard against the Picts and other Highland tribes. The Roman fleet was of course used at all times to guard the seaways between Britain and the rest of the Roman Empire, as well as to carry supplies along the coast when the army was fighting near by. This gave the Romans the usual immense advantage of sea-transport over land-transport, never less than ten to one and often very much more. The Romans could thus keep their army supplied with everything it needed. The Celts could not. Eighteen hundred years after Caesar's first landing in Britain, Wolfe, the victor of Quebec, noticed the same immense advantage enjoyed by King George's army over Prince Charlie's, owing to the same sort of difference in transport, King George's army having a fleet to keep it well supplied, while Prince Charlie's had nothing but slow and scanty land transport, sometimes more dead than alive.