One error brought the scheme to ruin--an error neither of strategy nor of conduct, but of scientific knowledge. John had miscalculated the time at which, on that night, the Seine would be navigable up-stream; and his counsellors evidently shared his mistake till it was brought home to them by experience. The land forces achieved their march without hindrance, and at the appointed hour, shortly before daybreak, fell upon the French camp with such a sudden and furious onslaught that the whole of its occupants fled across the pontoon, which broke under their weight. But the fleet, which had been intended to arrive at the same time, was unable to make way against the tide, and before it could reach its destination the French had rallied on the northern bank, repaired the pontoon, recrossed it in full force, and routed John’s troops. The ships, when they at last came up, thus found themselves unsupported in their turn, and though they made a gallant fight they were beaten back with heavy loss. In the flush of victory one young Frenchman contrived to set fire to the island fort; it surrendered, and the whole population of the New Andely fled in a panic to Château-Gaillard, leaving their town to be occupied by Philip.[423] The Saucy Castle itself still remained to be won. Knowing, however, that for this nothing was likely to avail but a blockade, which was now practically formed on two sides by his occupation of the island fort and the Lesser Andely, Philip on the very next day[424] set off to make another attempt on Radepont, whence he had been driven away by John a year before. This time John made no effort to dislodge him. It was not worth while; the one thing that mattered now was Château-Gaillard. Thither Philip, after receiving the surrender of Radepont, returned towards the end of September to complete the blockade.[425]