There's been alot of focus on this subject recently and a good deal of excellent information and insight. Clearly, no one can accurately know how
things will play-out after a 'Sit-X' event. And given that, everyone's castle situation is going to be different.
My hunch is that you're largely correct. Areas with the highest population densities (more urban) will deplete resources the fastest and as a result
begin displacing the earliest. As has been seen in other 'survival' situations (like Katrina), the displaced will first target commercial entities
for food, water and essentials. The displaced will begin to move outward, in a more-or-less concentric manner to find safety, shelter and supplies.
Any 'Sit-X' will quickly eliminate motorized travel. As a result, people will only be able to loot what they can carry. Anyone still able to drive
around (in order to loot at a wholesale scale) is going to attract a great deal of attention and interest.
As people move outward from the urban centers their density will drop as they spread out. Their ability to continue moving will drop-off after
prolonged exposure and fatigue sets in. Those that do make it into the suburbs will be relatively dispersed and poorly equipped. If organized and
equipped people are intent on your home or its contents you'll have a fight on your hands, no question. But I think the key is to make your home
less attractive than others. There are plenty of homes to loot. People will tend to gravitate to those that represent the least threat to them.
Everyone's 'castle' situation is unique and your plan needs to reflect that.

