There is more then one way into the BigDungeon and each has it's own obstacles and dangers to deal with.
The Grand Entrance
Once ages past a dwarven stronghold dominated the area of the Big Dungeon. The original guardians are long dead and factions battle for control of the grand entrance all the time. Force, cunning or negations may guide a party past the gate to the BigDungeon.
the ruined city that once sat outside the BigDungeon is mostly leveled but there still exists the subterranean edifices of the old city. Among a number of basements and cellars one can find the ancient sewers. Exploration of the sewers of the old city may eventually lead one to discover the underground canal that fed the cisterns and sewers of the old city. Following that underground canal to it;'s source will lead one into the BigDungeon.
Partway up the mountains the BigDungoen lays buried beneath one will find a grotto sometimes used as shelter and temporary pens of the goat-herders that dwell in the region. In the back of a small cave one can clearly see a great stone wheel door and behind that the BigDungeon.
Several streams pour down from the mountains above the BigDungeon. Sheltered beneath the cascading waters of a waterfall one can find entrance to the BigDungeon. One must only be able to discover which path leads to the proper watercourse to find entrance to the underworld beyond.
Not too far from the BigDungeon one can find an ancient shrine guarding a magical portal that leads to the BigDungeon. One must only learn the command words and deal with those that dwell in the area to gain access.
Here and there high above the BigDungeon one can find secreted shafts leading deep into the depth of the earth that allow those that dare the climb access to the BigDungeon.
An ancient tower may be spied high up along the slopes over the BigDungeon. Some rumor a wizard once dwelled in the isolated tower, surely there may be an entrance into the tunnels beneath if one dared to build a tower there.
The Cliffside Tombs
The ancestors of the goat-herders buried their folk high up in cliff side tombs. It is said some of these tombs lead deep into the cliff face and mystics once preformed unknown acts far beyond the eyes of men. Mayhaps these tombs can lead into the BigDungeon?
There are other possible entrances rumored to be scattered about the region old prospectors mines, well shafts, sinkholes and caves known to only a few.