You venture a ways down the trail that first lead you up to the encampment, and then off to a side with jutting rocks that would protect you from at least some of the wind and weather to set up your camp. There didn't appear to be anyone outside of their own tents in the makeshift village as you departed, though there were still people awake within some of the tents shuffling about.
This way, if anyone does come looking for you, they will at least find an appearance that you have left, but also bedded down for the night, so long as they don't look too close.
As you go off to find an alternate entrance to the fortress above, you find that the going isn't easy, particularly in the dark.
The stronghold was no doubt constructed where it was for a reason - it is basically unapproachable from all but one direction. You must contend with irregular rocky terrain, sudden elevation changes, and fields of loose scree that threatens to take you with it any time it begins to slide.
But it is possible to make your way around the backside of the peak to look for - what Eothain pointed out is - a pretty standard design feature for fortresses such as this; an emergency escape route or secret entrance in the case of a siege.
And while the going is slow and difficult, at about an hour before sunrise you find a cleft in the cliffs with the fortress looming high overhead, in which there is an ironbound, heavily rotted wooden door worked into the shadowed rock.
OOC:
As you're going the night without rest, please give me a Constitution DC11 save to resist taking a level of Exhaustion.
Also - fun fact - in the intervening years since we started this campaign - new rules have come out for Exhaustion. And we will switch to those (tho at 2 levels, it won't be any less punishing than the earlier ones).