Listen carefully. My lord does not trust the clan currently in possession of this fortress -- you can see the effort and expense he has gone to to assemble this mercenary force as a counterweight. Were you to expel the current owners and capture the fortress, I am sure he would be most grateful. (Of course, that probably won't last long -- it's not called a "3-day reign" for nothing...)
Oh, forgive my muttering -- just talking to myself, you know. In any event, let me explain a little about the two types of fortresses.
Territorial fortresses are built within a territory to defend against monster attacks, while border fortresses are established along the boundary between two territories. If you have a map, take a look for yourself and you'll see what I mean. Blue flags indicate territorial fortresses, whereas red flags indicate border fortresses.
Ask him how to conquer a fortress.