Lifted from Wikipedia:
Märzen has its origins in Bavaria, probably before the 16th century. A Bavarian Brauordnung (brewing ordinance) decreed in 1539 that beer may be brewed only between Saint Michael (Michaelmas, 29 September) and Saint George (23 April). The reason for this requirement was the increased danger of fire during the warm and dry summer months.
Over the summer months, beer had to be stored in a Lager (storage), in caves or stone cellars, sometimes built into the sides of mountains or hills. Frequently these locations were chosen because there was a pond nearby. During the winter, when the pond had frozen, blocks of ice would be cut and put into the cave or cellar. This was usually possible until March, when the beer was brewed to be stored there for months to come. The entrance to the cellar or cave would also be shielded from sunlight by planting Horse-chestnut or Conker trees in front of it, which have large leaves.
Some of these cellars and caves are still in use, primarily in Franconia, as part of beer gardens which are often still shaded by the traditional chestnut trees.
The original Märzen was described as “dark brown, full-bodied”. As intended, the beer was often kept in the cellar until late in the summer, and remaining bottles were served at the Oktoberfest. In order to last so long, either the original gravity and alcohol were increased or the hopping was strengthened.
The style is characterized by a medium to full body, a malty flavour and a clean dry finish. In Germany, the term covers beers which vary in colour from pale (Helles Märzen), through amber to dark brown (Dunkles Märzen). Brewers in the Czech Republic also produce pale, amber and dark beers in the Märzen style, called respectively 14° Světlé Speciální Pivo, Polotmavé Speciální Pivo and Tmavé Speciální Pivo.
The Austrian style closely resembles a Helles in color, body, and flavor balance, and is the most popular beer style in Austria.
Common names for Märzen include Märzenbier, Wiener Märzen, Festbier, and Oktoberfestbier.
Moa’s Märzen is more on the amber side, is light and easy going.