Gary Fagan is a music teacher and composer in Virginia. He has this to say about The Gates of Destiny:
This exciting work for beginning band uses a technique in which primary themes begin on a unison note, and then move stepwise away and back while the other voices remain static or proceed in contrapuntal inversion. This creates chords that expand and contract almost continuously beneath the melody. The percussion section plays an important role providing a rhythmic base which enhances the flow and development of the piece from beginning to end.
Or, to put it in simpler terms, Fagan builds the main melody by having one instrument play little bits of a scale while other instruments repeat the first note. This gives the music a sense of expanding and contracting as Fagan says. Listen to it here.