Essentially a fortress hub is any airport where one airline has a dominant number of gates in use, totalling more than a 70% majority.
Competing airlines are allowed to fly from such an airport, however in reality the hub airline has such a distinct monopoly over resources that circumstances can become anticompetitive. For example, the main airline negotiates contracts with the catering contractor, the baggages handling contractor and is also a major employer within the airport. Therefore this influence can be leveraged to the airline's advantage by influencing decisions which would not usually be within the scope of effective airline control. For example; future expansion and infrastructure developments within the fortress hub.
A case study of such a fortress hub, is Detroit Metropolitan Wayne County Airport (code: DTW / KDTW), found in Michigan, Detroit.
In the 1980's several smaller carriers endured mergers and acquisitions, for example Republic Airlines merged with Northwest Orient Airlines, which later became Northwest Airlines.
Fast forward to 2008 and Delta Airlines bought Northwest Airlines to form the largest carrier in the world, employing in the region of 75,000 staff.
These airlines subsequently became part of the SkyTeam alliance and now have a overwhelming presence at Detroit Airport, due to the scale of the combined members; Delta Airlines, Northwest Airlines, and Air France occupying almost every air-side gate. This would have been even more impressive but for fellow SkyTeam member Continental Airlines, also flying from DTW, departing this month to join the competing Star Alliance.
Technorati tags; Detroit, fortress hub, DTW.