Most easy way is from the city of Trondheim. If you have a car, you can not read more. If not, then you'll need to catch a morning train at the central railway station of Trondheim and ride for about an hour to Hegra stop (Free train timetable at www.nsb.no).
There is a 3-km asphalt road up to the fortress from railway station in Hegra village.
It should be noted that there are only two trains to Hegra available in timetable, one around 8 am and back around 5 pm and it all. Other public transportation is quite insignificant, because almost all locals here have cars and do not needs in buses. I missed the last train to Trondheim and was getting back by the chaise-type school bus in Stjordal.