Fortified means strengthened or protected. When you've taken lots of vitamins, you should be fortified against getting sick!
A fortified place is somewhere, like a military fort, where you are protected from danger, perhaps because you are high up on a hill or because your walls are built of iron. A fortified outfit would be a suit of armor. Fortified can also describe foods that have been supplied with added nutrients. Fortified milk might include added vitamin D, to ensure that you are getting your recommended supply.