Weaving through a story that is underweight for a novel, I introduce a breakfast scene where a woman drinks tea. Hmm. Tea. From overseas, no doubt. Which raises the question of where the port is -- and while the main characters can't leave the city for plotting reasons, there's no reason why the city can't be a port. It's a capital, but capitals have a tendency to be ports, actually. Or at least, it's not uncommon, since I haven't actually done a statistical analysis.
And once it's a port, I can easily send my characters wandering down there, especially since there's a magic afoot in the city that can dump them in locations they weren't intending while they are running away from something more unpleasant than being dumped in a random location.
With any luck, the novel grows heavier.