Marstal is a town in southern Denmark, located in Ærø Municipality on the island of Ærø. Marstal has a population of 2,077 and is the largest to…Marstal is a town in southern Denmark, located in Ærø Municipality on the island of Ærø. Marstal has a population of 2,077 and is the largest town on Ærø. It was the municipal seat of the now abolished Marstal Municipality.