I ran this as a kind of a "get to know AD&D" adventure for a friend and found it to be perfect for it.
The encounters were a good enough challenge for a lone first level character. They were not overwhelming and each proved to be a danger to contend with for the heroic mage/thief. I imagine a pure mage would of had terrible time in the dungeon, however, without the sneaking and backstabbing from the thief side.
I also found that the adventure was easy to slip into basically any campaign universe as a good first step for a solo adventurer. In my case I used the Village of Hommlet to introduce as a starting village and Fortitude Dungeon was a good fit for a forgotten hillside outpost some miles away from the village itself. Including character creation and the before and after dungeon village events we ended up clocking around ten hours of AD&D goodness out of this adventure. For the dungeon part itself, I think we spent somewhere between three or four hours exploring it one careful step at a time.
Bottomline is, I would recommend this adventure for a good solo adventurer experience and it fits as a perfect bridge for bringing my friend towards future adventures set around Hommlet, some of which might just be other FS series adventures.