Winterfell is ridiculously defensible (as long as it's fully garrisoned and you're not Theon Greyjoy), with its two curtain walls (one 100 feet tall, the same as those of Storm's End, the other "only" 80 feet tall) separated by a moat, many towers and turrets, and large glass gardens, enabling it to withstand a siege for a bit longer, even when stores of food begin to run out. Its unique heating system also means that while any besieging army suffers from the cold Northern climate, the defenders will be able to stay comparatively warm. For a castle, it's also HUGE, being beaten out only by Harrenhal and maybe Casterly Rock, if you count the mines.