1) The chair for a person of high rank, thus Siege Perilous, the one unoccupied seat at King Arthur's Round Table, to be occupied only by the one who would achieve the Grail.
2) A military siege involved surrounding a town with the intention of starving the inhabitants into surrender; later, a privy. Orig. a siege was the place where herons waited for prey to approach.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.