Archeobotanist Hans-Peter Stika, of the University of Hohenheim has been looking at ancient Celtic sites, trying to figure out how they made beer.
It seems they dug a ditch and poured barley and water in it, leaving it to sprout. They then dried the barley with a fire. The beer was then flavored with gruit that contains yarrow, carrot seeds, mugwort, and henbane. The beer was then heated slowly by holding the it over a very low fire and adding yeast. Fruits or nuts was then sometimes added. When it was cooler, it could finally be consumed.