Explain the importance of siege warfare in the High Middle Ages. What was the role of the garrison and castellan?

From the onset of the eleventh century, the world was a mass battlefield, and it was the castle that evolved into the mainstream method of fortification and defense. So easily constructed and so simply adequate, the castle came to signify the opening move in the real-world game of chess that prevailed in the later middle ages. Because castles were so adept at fending off attacks, and the high cost of both maintaining a field army …

…were common and other occasions when an attacking army was able to simply "walk in and take over" (Bradbury 75). However, when the threat of danger presented itself, the number of men consisting of a garrison could be overwhelming--such as the 500 men-at-arms mustered at Rostate to defend against Barbarossa (Bradbury 74-76). RELATED ESSAY (kind of a two-parter with this one is: Identify the various siege weapons used in medieval warfare. How were each employed in sieges?