maerwald?

Doubt it (I've picked a couple different options, but not all), but if you don't kill him, you can't be railroaded into 'owning'* the stronghold (whether you do anything with it or not).

 

*you'd think the duc might object to some random foreigner fortifying and repairing a stronghold roughly a day from the capital in this midst of, well, all sorts of craziness and instability. They might well be up to something...

Oh, what do you mean they've also killed the rightful* ruler of the area northwest of the capital as well?

 

*crazy as a box of frogs, but assassination of nobles is generally frowned upon.