With the recent move by Southwest into DEN, there aren't a lot of other places I consider "fortress hubs" left where the main carrier has real pricing power.

SFO - WN was there but weather kills their operations.
EWR - CO has taken every gate plus some to insure no competition.
DCA - I consider DCA to have "fortress hub" pricing power, but for totally different reasons than any other airport, except maybe LGA.
MSP, MIA, IAH (the latter two solely for the international traffic) could also be argued, though I don't put them in the same category.
Others?

CLT - how does US hold onto this one so well? Do they grease palms, agree to keep X number of jobs in the area, have lock-solid gate agreements? Whatever they have been doing, it has worked, but does anyone know the inside story?