I just had a similar problem with Deorcnysse. I met up with Umber early in a new game and was surprised that their power score was 120+ when everyone else was just hitting 40 at best. They declared war on me within about 20 seasons, and luckily I had a strong champion scouting close by at the time and conquered their closest city, which happened to be Deorcnysse (it had quite a few darkling defenders inside as city militia -- about eight (8) IIRC, and the battle was pretty hard). My power score shot up and Umber's plummeted immediately once I owned the city (now I'm at 120 and they are down at 30). I was very surprised after reviewing the city's details to see it listed as size 9 (how is that even possible?), since I didn't know it was a special quest city until I did a bug search on the forums and found this thread and others. It appears that population size is playing havoc with the scores, or else it really is a very powerful city that is too easy to take as per the OP. Regardless, something is not quite right about it.

Running version 1.3 of Legendary Heroes.

I'm pretty sure it's not supposed to specialize.  Other than that, IDK, I've only conquered it once (it just isn't near me in most of my games).