One of the major features in Weaverham is the road West. The discovery of the route west through the mountains was one of the defining features of the town. When I came up with this distinctive feature, there wasn't really anything on the other end of that route, it just existed as a point of interest. Now, the players haven't questioned this; however, it's not something that could stand for long. So the Walled City of Mirandir came into being as something Worth Going To, in this case, a strange city with a lot of wealth and access to exotic stuff, justifying the existence of trade caravans.
Then, there was another issue. Considering that there was a medium-sized city on a major river flowing west just a few days march south of Weaverham, why was that route through the mountains so important, especially considering that it was only open during the non-winter months? Why wouldn't merchants just jump in their boats, sail down the river, then cut north back up the coast to Mirandir?
The answer clearly was that there was something bad in the way, something terrifying enough that only the extremely foolhardy would risk sailing that route. Now, that something could be on the coast itself because the current state of sailing in the Weaverham campaign is such that few people have the technology or the cojones to sail out of sight of land.
Originally, the answer was: pirates. An established camp of them preying on water traffic -- everybody likes a pirate city and that would certainly "work" within the established world. That doesn't really have the "oomph" to it that I'd like, especially if we were talking about something that had been around for generations.
Add in another bit of world background, the Hyperborean ruins scattered around, roughly equivalent to those left behind by the Afternoon Cultures in M. John Harrison's Viriconium series. In this case then, there is a port/castle complex on the north lip of the river entrance, mostly destroyed; however, some towers and halls still remain and in them dwell a clan of mutated pirates, warped by their surroundings and granted power due to their possession of what they refer to as a "firelance", a cannon mounted in one of the towers and capable of splitting ships in half from miles away.
- Since few, if any, ships dare to sail past their hideout, the pirates make their bones by raiding Mirandir and the shipping going into the river mouth (the south part of the river delta being outside of the range of the firelance). They strike quickly and then retreat.
- The mutating effects of the ruins have the effect, aside from making the pirates gruesome in appearance, of rendering the inhabitants sterile. The pirates then rely on increasing their numbers through abductions, either through their shipping raids or attacks on nearby settlements. For obvious reasons, children are preferred targets, although young males work well too.
- It's probable that the pirates have some way of dredging stuff off the ocean floor. Maybe some kind of Hyperborean submersible. Maybe some of the mutants have gills. Maybe they have a deal with some undersea dwellers who give them a cut of the detritus that showers onto them from above. Lots of possibilities.