According to The Free Dictionary, fortify means to make stronger. Fortified means that something is already strong and is the past tense form of fortify.
According to The Online Dictionary, there are several definitions for the word fortified. In general, fortify means to make something stronger. However, there are other acceptable uses of this word. Fortify can mean to reinforce by adding materials. An example of this is, "The fence was fortified with thick wooden beams." There's also an acceptable use of fortify that describes a person or people. In this instance, it means to give someone physical strength or endurance. An example of this use is, "She fortified herself with a good breakfast."