One of the adventures in the recommended Fantasy Hero Battlegrounds book is a castle besieged by an army of monsters. This adventure is called Eisburk-Beyond-the-Shoals, and it involves an attempt by the Orcs and Goblins to break into the chapel of the keep to steal an ancient cornerstone that is part of a dark and evil cult. This running theme of collecting evil pieces for an unnamed evil goes through the whole book, but if the GM wants a different, more extensive adventure then this add-on can be used.
This module is an adaptation of an old contest-winning module that was in Dragon (Issue 78 - Oct 1983) entitled Citadel By The Sea. That module involved a castle and Orcs, but it was instead a deception by the Orcs involving a ruined keep and an attempt to recover an ancient artifact.