The software fortress is the basis for the software fortress model, so I'll start by defining that:
Definition: Software Fortress
A software fortress is a conglomerate of software systems serving a common purpose and typically owned by a cohesive group of individuals. These software systems work together in a tight trust relationship to provide consistent and meaningful functionality to a hostile outside world.
As you can see, the definition of a software fortress includes both technical and organizational aspects. From the technical perspective the software fortress is a collection of systems. This collection can consist of many different types of software systems, including but not limited to processes, system services, component-packaged ...