Above all, remember this: The fortress represents a trust boundary. This is the fundamental principle of software fortresses. The fortress represents not only a technical trust boundary, but usually an organizational trust boundary as well. Here I will focus on the technical idea of the trust boundary, formulated as the trust rule:
Definition: Trust Rule
Everybody inside a fortress trusts everybody else inside the fortress, and nobody inside the fortress trusts anybody outside the fortress.
The trust rule tells us that within a fortress there is complete trust, kind of a technical utopia. This concept is perhaps the most radical idea of the software fortress model. Consider, for example, a data strongbox ...