A Mighty Fortress most often refers to A Mighty Fortress Is Our God, a 1529 or 1531 hymn by Martin Luther.
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Track 10, listed as Pachelbel's Canon in D is in actuality Ein feste Burg ist unser Gott by Martin Luther, or as it is known in English, A Mighty Fortress is Our God.
His setting of Martin Luther's words Ein feste Burg ist unser Gott (A Mighty Fortress Is Our God) is still regarded by the Germans as their representative national hymn.