Some uncertainty attaches to the rare remains of fortifications of _burhs_ “wrought” by Saxons and Danes. It would seem that they cannot have been very strong. The defences consisted of the usual earthen bank with a stockade on the top and an outer ditch; but one may safely assume that the strength of the defence lay mainly in the actual stockade, and that the bank and ditch never reached formidable proportions. Thelwall, near Warrington, takes its name from the wooden stockade, the wall of thills or upright palisades with which Edward the Elder surrounded the village in 923.[30] There are exceptional cases in which we hear of a stone wall; but in these instances the _burh_ was a Roman city or station, and the wall was a Roman wall. This may be fairly assumed with regard to the wall of Edward the Elder’s _burh_ at Towcester (921). It was certainly the case at Colchester, where the Danish defenders were worsted in the same year by the _fyrd_ of Kent and Essex. When Alfred the Great “repaired Lundenburh” in 886,[31] he undoubtedly made good the weak places in the stone wall which the Romans had made round their city of London. The _burh_, the fortified stronghold of a Saxon community, comes into prominence as the result of the Danish invasions of the ninth century. The method of the invaders was in almost every case the same. Seamen before everything else, they sought in their long ships the estuaries of rivers, and proceeded to penetrate inland as far as the stream would take them. From a base of operations, preferably an island in the river, where they could harbour their boats safely, they rode into the surrounding country, burning and pillaging. In 835, allied with the Britons of Cornwall, they came up the Tamar, and fought a battle with Egbert at Hingston down, west of Tavistock, in which they suffered defeat.[32] In 843, they effected their first permanent settlement in France, on the island of Noirmoutier, south of the estuary of the Loire: they invaded the banks of the river, sacking Nantes and killing the bishop, and, after their summer campaign was over, settled down to build houses for winter quarters on their island.[33] Each of the great French rivers was infested during the next few years by bands of northern pirates. Northmen in 845 sailed up the Garonne to Toulouse, and up the Seine to Paris, where destruction was avoided only by buying them off. Towns which lay near the rivers or sea coast were invariably sacked. Sometimes the pirates, growing bolder, left their ships and rode for some distance inland. In 851, starting from Rouen, they pillaged Beauvais. In 855, after burning Angers, they took to the land and sacked Poitiers. In both cases, however, their return journey was successfully cut off by a French army. In 856 the Danes of the Seine made their winter quarters at Jeufosse, on the bend of the river between Vernon and Mantes, and within no great distance of Paris.