These castles depended not only on their walls for defense, but were often built on hills or mountaintops, to further frustrate an attacking army. The other obvious difference with the European model is the wood structure perched on top of the wall. From a formal standpoint, i love the differentiation of the occupied space, in wood, from that of the "earth", in stone, if for no other reason than its articulation of the Japanese cultural relationship to nature, which i think is summed up in the picture above, where nature is understood not as something to be dominated, but to exist alongside as an equal. The delicacy of the wood structure sitting on the stone wall is telling.
i believe this castle was only used for ceremonial purposes.