You just described the exact situation we encountered as PC's in the opening scenarios of diaglo's OD&D campaign (extremely modified B-1, B-2 + a horde of originality). In the end we escaped mostly intact but the Keep disappeared into a giant sink-hole. Later we probably could've put ourselves, as PC's, into a similar situation but we declined and suffered huge losses trying to mop up the siege mess after the fact. Currently we once again find ourselves in a similar situation, holed up in a wooden fort/villa, probably about to be besieged again. From a player's point of view this is an exciting and perilous proposition. There is a very fine line between knowing when to "hold 'em" and when to "fold 'em", when to walk away, and when to run.