If you add nutrients to something you fortify it. Food scientists have found ways to fortify cereal, but in addition to vitamins C and D, they usually add a lot of sugar.
If you strengthen your defenses, you fortify them, perhaps by adding more arms or increasing your defensive walls. Sometimes the defenses you fortify will be tangible and sometimes the word is used metaphorically to mean give courage. If a castle was originally vulnerable to attack, the owner might fortify it by adding a moat — and maybe filling it with crocodiles. If you're nervous about giving a speech, you should fortify yourself with a pep talk first.