How to Make Celtic Beer

Archeologists excavating the 2550-year-old Celtic settlement of Eberdingen-Hochdorf in southwestern Germany have discovered a brewery. The ancient Celts would dig a ditch, put in water and barley, and let it start to sprout. They would light fires at both ends of the ditch to dry out the now malted barley, giving it a smoky taste, and causing the production of lactic acid, souring the malt. The grains were boiled by adding hot stones. (The articles I have found on this are unclear. Apparently, the fires dried the grains, but didn't evaporate the water. More likely, they added more water to the dried grains. I will keep investigating.) Since there were no hops, herbs like yarrow, carrot seeds, mugwort, and henbane were added to flavor the beer, technically called gruit.  The beer was fermented with wild yeast, probably from the skin of fruit, and was served at room temperature, obviously, and unfiltered.  Celtic beer was described, according to one source, by the Roman Emperor Julian as tasting like a billy goat.

Sources:
http://www.popfi.com/2011/01/19/iron-age-beer-from-ancient-distilleries/
http://io9.com/5734276/the-recipe-for-2500+year+old-celtic-beer