This paper is a tutorial on the proven but currently under-appreciated security mechanisms associated with "tagged" or "descriptor" architectures. The tutorial shows how the principles behind such architectures can be applied to mitigate or eliminate vulnerabilities. The tutorial incorporates systems engineering practices by presenting the mechanisms in an informal model of an integrated artifact in its operational environment. The artifact is a special-purpose hardware/software system called a "Guard" which robustly hosts defensive software. It is hoped that this tutorial may encourage teachers to include significant past work in their curricula and students who are self-teaching to add that work to their exploration of secure computing.