[Verjigorm]

I have used them with Man-std (bad results with LotT, good KoA production) and Marignon-std (Good results when I had a good starting prov) and Arcoscephale-GE (to help counteract forced sloth in an unsuccessful attempt to make armies of Arco-GE units). Their effectiveness depends on what starting location you draw, but you'll end up with a little bigger army of strong national troops. I would say the only good use for it is squeezing a little extra resources out of your main at the expense of being able to expand your unit production over a large area. You also cannot place them within 2 squares of each other because if they share a square that you want to produce something on (e.g. a sage or something) the two adjacent fortified cities will consume all of the provinces resources. It's a very restrictive castle and I usually just go with the Castle or Fortress if I need a little extra production boost. My brother uses FC with Pythium on occasion to increase his emerald guard production. You have to be very picky about your new fort positions, but they pay off pretty well if you pick a nice mountainy/foresty area.