Regarding reputation, it depends on the faction. For example, Inferno - Blood.
I developed Irina along the Blood path.
In my opinion, it's a very difficult mission. I replayed it about ten times, and that's no joke.
The Fortress is located: a little bit from the starting point, run down to the gates guarded by neutrals. From there, run to the right and pass through a narrow passage. (There's also a crystal mine there).
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