The great majority of mottes in England are planted either on or near Roman or other ancient roads, or on navigable rivers.[222] It was essential to the Norman settlers that they should be near some road which would help them to visit their other estates, which William had been so careful to scatter, and would also enable them to revisit from time to time their estates in Normandy.[223] The rivers of England were much fuller of water in mediæval times than they are now, and were much more extensively used for traffic; they were real waterways. When we find a motte perched on a river which is not navigable, the purpose probably was to defend some ford, or to exact tolls from passengers. Thus the Ferry Hill (corrupted into Fairy Hill) at Whitwood stands at the spot where the direct road from Pontefract to Leeds would cross the Calder. It was probably not usual for the motte to be dependent on a stream or a spring for its supply of water, and this is another point in which the mediæval castle differs markedly from the prehistoric camp; wells have been found in a number of mottes which have been excavated, and it is probable that this was the general plan, though we have not sufficient statistics on this subject as yet.[224] Occasionally, but very rarely, we find two mottes in the same castle. The only instances in England known to the writer are at Lewes and Lincoln.[225] It is not unfrequent to find a motte very near a stone castle. In this case it is either the abandoned site of the original wooden castle, or it is a siege castle raised to blockade the other one. We constantly hear of these siege castles being built in the Middle Ages; their purpose was not for actual attack, but to watch the besieged fort and prevent supplies from being carried in.[226] Hillocks were also thrown up for the purpose of placing _balistæ_ and other siege engines upon them; but these would be much smaller than mottes, and would be placed much nearer the walls than blockade castles. The mottes of France are in all probability much more decidedly military than those of England. France was a land of private war, after the dissolution of the empire of Charlemagne; and no doubt one of the reasons for the rapid spread of the motte-castle, after its invention, was due to the facilities which it offered for this terrible game. In England the reasons for the erection of mottes seem to have been manorial rather than military; that is, the Norman landholder desired a safe residence for himself amidst a hostile peasantry, rather than a strong military position which could hold out against skilful and well-armed foes.