There is no better example of the English medieval military mind at work than Edward I's great fortresses, built around North Wales at the end of the second Welsh war. There they still stand as masterpieces of military engineering, and eloquent witnesses to the Maginot mentality. Certainly they all had harbours, which demonstrates a foolhardy confidence that no enemy ships could intervene to frustrate their resupply by sea - but if Edward I had had any real conception of the flexibility conferred by sea power, he would never have built them. A castle filled many roles, as a symbol of power, a residence and administrative centre, but its military function was, in the modern jargon, as a 'force-multiplier'. High initial investment bought long-term economy by allowing a small force of cavalry to dominate a wide district. If an enemy appeared, a well-supplied castle could be defended for months by a small garrison, and the difficulties of feeding an army, stationary for a long period, were such that the besiegers were as likely to be starved out as the garrison.
Little of this applied in Wales, where small cavalry forces were very vulnerable and large bodies of infantry were needed to campaign among the forests and mountains. Since small garrisons were impotent and the Welsh were indifferent to siegecraft, the new castles did not even serve to tie down the enemy. Many of them, moreover, were built in quite the wrong places; the West Coast castles of Harlech, Criccieth and Llanbadarn were as far away as they could well be from points of strategic importance. Only Conwy could clearly be justified, as commanding the important position of Penmaenmawr which otherwised blocked all advance along the north coast, as well situated to be the English capital of North Wales, and as relatively easily supplied from Chester and Dublin. The value of the others was more symbolic or political than military.The Welsh rebellion of 1294 revealed the true worth of these symbols. It has been remarked that the existence of the castles made sea power 'more important', what this means is that most of the naval and military strength urgently needed for the recapture of English Gascony had to be diverted in a desperate and only partly successful attempt to save the king's white elephants from slaughter; the entire overseas empire was put at risk to preserve garrisons of 26 men in Harlech and 30 in Criccieth.