The Festungsberg is the hill on which the Festung Hohensalzburg (aka "Salzburg Castle") rests. It is considered to be one of the "Stadtberge" (city hills) and geologically speaking, one of the northern-most outposts of the Alps. The Festungsberg is 542 metres high and characterised by the castle (surprise), many very expensive villas and forest. This has not always been the case: For centuries, the southern flanks of the Festungsberg were used for growing vines.
The terraces of the vineyards can still be seen by those ruthless individuals that hike the forest in this part of the hill, and a guide book from 1925 reports that one could still find the odd surviving vine from Baroque days between the trees. For some peculiar reason, the cliff with the nunnery of Stift Nonnberg is usually considered to be a separate hill - the Nonnberg.
The Festungsberg has been populated since the Neolithic Age, when the neighbouring Rainberg was a major settlement. Later, Celtic tribes and then Romans lived here. The Festungsberg remained a rather insignificant part of Salzburg, though, until around 700, when a small castle was built. This tiny structure was extended to a proper castle in the course of a power struggle between Pope and Emperor in 1077 (Investiturstreit).
Along the southern base of the Festungsberg, you will find one of Salzburg′s most exclusive residential areas; a blend of Medieval, Baroque and 19th century villas. Didi Mateschitz, the founder of Red Bull and owner of Hangar-7, owns a house in the Brunnhausgasse. This lane is nice for walks and a good link from the Nonnberg to the Mönchsberg if you want to avoid the Altstadt side of the hill. Especially the southern flank of the Festungsberg is an important ecosystem for many local species. It is not really accessible, though.
Festungsberg on Wikipedia