The location of Farthen Dûr was the largest and most complex of the various set locations in the Eragon production. The set was located in the inactive remnants of a volcanic crater, called Saghegy, or Sag Hill. Well beyond the ten-to-twenty mile comfort zone from Budapest, the production team took over a nearby hot springs hotel and filmed at Sag Hill for the duration of the autumn months.

The Battle of Farthen Dûr, originally intended for daytime filming, was changed half-way through principal photography to become a night shoot. New challenges were presented with that change. More equipment needed to be hauled in and the crew and cast had to adjust to the freezing Hungarian nights.

At the end of November, the production finished their final shot for principal photography just as the winter snows began to fall.