Heaven City is precisely using this method to cut off other city-states' supply lines from the land route.
Thus, only the Goblin Commerce Association, which can circle to the south of the desert, can sell goods to other city-states. The goblins' greed allows them to squeeze every resource from other city-states. Heaven City, with a continuous influx of cheap commodities, gains a considerable advantage in trade and warfare against other city-states.
Now, Heaven City suddenly turned against Rock Cellar City and directly annihilated the city... there are only two possibilities.
Either Rock Cellar City was too greedy, took too much from the trades, manipulated too much, and Heaven City finally could not endure it anymore. So they plan to eliminate them and then replace them with their own people to oversee the trade.
Or, Heaven City's greed has surpassed the need for imperfectly obedient partners.
Regardless of the reason, it actually indicates one thing:
