Here's a different perspective.
Definitions of "fortress" and "undesirables" are fairly subjective. "Fortress" areas (25% of the housing stock) are generally expensive, have low crime levels and good schools. "Undesirables" (also around 25%) have problems with crime, poor schools, or both. For the purposes of constructing this graph, they are defined as follows.
Maps are generated using http://www.usnaviguide.com/zipunion.htm.
