Why Was the M&B Castle Construction so Popular and Effective in the Eleventh Century Warfare?

Who built castles and why? Where were most castles built?
Castles, for the most part, began to appear in the form of the Motte and Bailey (see next Q&A segment). The appearance of this type of castle coincides with the territories in which the Viking attacks were most prominent; therefore, strengthening the argument that castles were erected as a quick form of fortification against an enemy who lacked siege engines (Bradbury 51). Admittedly, castles were erected for defense, and in this capacity would simultaneously protect the villagers who lived within its borders and control the neighboring territories, serving as a combination command post, defensive fortification, judicial authority, and a symbol of power (Contamine 45-46). However, save for the M&B castles, the majority of the development of castles came in the years after the Viking raids subsided (Bradbury 52). It is, therefore, necessary to explore other reasons for which castles were built.
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