Dion says, in brief, that the Britons crossed the Thames near where it enters the sea. They did so easily ‘because they knew the firm ground ... and the easy passages, ... but the Romans following them came to grief at this spot.’ We can hardly imagine the Thames as fordable anywhere below London, and the probable meaning of the passage is that the Britons traversed the marshes by well-known tracks, and then crossed the river in boats or on rafts. Now comes the most curious part; and, if Dion could be relied upon, we have an invaluable reference to the earliest London Bridge. The Celts, he says, again swam the river, and other troops forced a bridge a little way upstream. We can hardly imagine a bridge existing below London, and if it stood anywhere, it would certainly be at the one place clearly marked out by Nature for the passage across the river of the road from the south-east to Verulam. London was such a remarkable road centre in Roman times that we cannot easily believe that it was not so long before, and the construction of a pile bridge was certainly not beyond the resources of a powerful ruler like Cunobelin, who had, without any doubt, skilled foreigners at his disposal, besides abundance of unskilled labour. Moreover, it explains satisfactorily the importance of London, which Tacitus describes as a great trading centre only eighteen years later. Whatever his authorities, Dion’s account of the operations, studied with the aid of the map, is logical and clear. The Britons are driven back from the Medway, and retreat to the estuary of the Thames across dangerous marshes--that is, we can hardly doubt that Vespasian’s turning movement cut their line of retreat on London, and they were forced to fall back northward by Higham into Cliffe Marshes. The Roman pursuit was checked by the difficulties of the ground, but Plautius was between the Britons and London. He therefore marched for the bridge. The Britons, hurrying from Tilbury Marshes, reached London too late to destroy the bridge or occupy it with more than a fraction of their forces, if indeed at all. The bold action of the Batavians, who accomplished a more difficult feat than the swimming of the Medway, coupled with the seizure of the bridge, forced the Britons to abandon the defence of the Thames.