Wood castles constructed throughout the land were useful in their time and were the preferred method of defense in newly conquered territories. Their advantages were economy and speed of construction. Their disadvantage was their comparative vulnerability.

See History 3 for more about these wood castles.

The proliferation of castles differed in each country. In Germany the spread of castles came somewhat later than in France and the impetus was a conflict between church and state, and the exploitation of this conflict by anyone with the wherewithal to fund such an expensive undertaking. In Charlemagne's empire his bureaucratic system was successful in maintaining relative stability. But in virtually all succeeding monarchies the lack of such organization necessitated implementation of a patch work of royal supporters to protect citizens and royal lands. Feudalism, for a while, filled the bill. With feudalism a trusted knight or noble would be granted land, often including many villages, in return for services to the upper lord or monarch. This service included the obligation to provide agricultural products as well as a certain level of military capacity with which to accompany the lord or monarch on campaigns or relieve him when threatened or besieged by an aggressor. A noble or monarch without such lands to grant had little with which to bargain and assure loyalty.

In the 11th century Henry IV began an aggressive campaign to subjugate the Saxons. Saxony was critical to a youthful monarch in need of more royal lands and sources of revenue to pursue ambitious expansion plans. Saxony was also rich in silver ore which he intended to hold at all costs. But Henry was left with a monarchy that was land poor as a result of major grants of land made by those entrusted with the kingdom until Henry came of age to rule on his own. In Saxony Henry could confiscate lands based on accusations and judgements, either true or trumped up, against landed nobles. These lands were then secured with a castle and manned by or granted to trusted knights from outside of Saxony. This draconian approach led to uprisings by the Saxons whose history of resistance always made them a difficult people to subjugate anyway. Before Henry could entirely quell his troubles with the Saxons his long running quarrel with the papacy, under the lead of Pope Gregory VII, began to absorb the king's energies and decimate his base of loyal and obligated lords and nobles.

The Investiture Contest was the struggle between lay nobles and the church over the power to appoint high church positions and grant lands and power to church officials in return for loyalty to the lord, just as with any enfeofed knight. The problem lasted a good four decades and resulted in frequent political and military skirmishes without ever resolving the issue in Henry IV's reign.

The pope not only declared Henry's crown null and void, he excommunicated the king and gave license to all nobles with sworn allegiance to Henry to dismiss any and all obligations to the crown. Ambitious nobles leaped on the opportunity to launch a castle building frenzy that would have been illegal earlier. Many did so because it became necessary, under such anarchic conditions, to take the defense of their lands and villages into their own hands. Yet others did so in defiance of the crown and in hopes of using their new freedom and powers to enrich themselves and expand their own land holdings through unrestrained aggression against neighbors that would otherwise have had the protection of the crown. The results of this era and of the next few centuries are a country dotted with thousands of castles, fortified manors, village defenses and castle ruins.