In war times, as well as in peace, Cities and Castles stock great amount of food. Let it be for any unforeseen shortages or for a possible attack / siege. Such stockpile may provide the necessary nutrition for the garrison and its population. Thus, it may take some good amount of time for a Castle or City fall, due to its population and garrison, starving. Plus - in areas that are hit by enemy forces, where the fields are burned down or in steppes / hills / mountains / snow plains, it is really hard for the attacker to gather enough food for its army - unless there is a continuous trade from other Cities / Castles to the army that sieges the fief.
Apart from the possible trade, the fief loses nothing. Every property that is important to the fief itself is inside its wall.
Also I don't understand why would you lose your equipment. Would you eat a bastard sword? Siege ladder?
If this should have any actual purpose while being realistic, there should be an additional stock line. Only thing you should the Siege effect is that and that only.
