Sentence:  For security's sake they [the Germans] gathered together in villages and cities. These they surrounded with heavy walls and towers, and protected them by castles, erected on steep cliffs and mountains. The custody of these <WSD>strongholds</WSD> was entrusted to the most efficient warriors, who in time formed a separate class, the nobility, from which the heads of the whole nation, the princes, kings and emperors were chosen. 
Meaning of the word marked with <WSD> tags:  A place built to withstand attack; a fortress.