(verb)
to strengthen a place and make it secure and defended against attacks, by installing protective structures such as forts;
e.g. The army began to fortify the city in preparation of the invasion.
(verb)
to enrich food and increase its nutritional value by adding substances like vitamins or minerals;
e.g. The process is intended to fortify the milk with more vitamins.
(verb)
to add distilled alcoholic spirits to wine during the fermentation process in order to make other drinks (such as sherry or port) and/or to increase its alcohol content;
e.g. You must fortify the wine with brandy before using it in this beverage.