Between the time of the formation of this little collection of huts with its slight protecting stockade and the coming of the Romans much happened. The Ancient Britons learned to make roads—primitive ones, of course—and in all probability they learned to make embankments to the River. Their greatest trade naturally was with Gaul—France, that is—and also, equally naturally, practically all such trade had to come through the one most suitable way, the spot which has always, through all the ages, been the gateway into England—Dover. In the days when sea-going craft had not reached a high stage of perfection it was necessary to choose the shortest passage across the channel, and, though no doubt other ports were used, undoubtedly the bulk of the merchandise came across the narrow Straits. This meant, without a doubt, an important road going north-westwards towards the centre of England. Now right across the country, from west to east, stretched the great natural barrier, the River, effectively cutting off all intercourse between the south of England and the Midlands and north; and at some place or other this road (afterwards known as Watling Street) had to cross the barrier. It was inevitable that the spot where this crossing was effected should be, both from a military and a commercial point of view, a place of the very greatest importance. In the earliest days the road skirted the south side of the marshes facing Llyndin, and passed on to the ford (or ferry) at Westminster, and thence on to Tyburn. But Llyndin was growing in strength, and the need of a lower crossing was probably soon felt by the inhabitants of the little hill. Now lower crossings of the River were by no means simple. As we said just now, right from the mouth westwards till we reach the spot where London now stands there was simply a great collection of marshes and fens. Here and there, on both banks, tiny patches of firmer soil jutted out from the impassable wastes—the spots where Purfleet and Grays now stand on the north side, the sites of Gravesend, Greenhithe, Erith, Woolwich, and Greenwich, on the south side; but in each of these cases the little gravel bed or chalky bank was faced on the opposite shore by the dreary flats (an ordinary natural happening caused by the washing away of the banks, to be seen in any little stream that winds in and out), so that never was there any possibility of linking up north and south.