The Discworld Mapp places Ecalpon on the far side of the main hub continent from Ankh-Morpork, close to the cold frozen lands and the icy sea that separate the Counterweight Continent from the rest of the disc.
In Discworld Noir it is also discussed by the conspiracy-mad priest Malaclypse who claims that the city is the location for secret trade talks between Ankh-Morpork and the Agatean Empire due to it being the closest port to the Counterweight Continent. While this could all be Malaclypse's ravings, some of what he says is true, and it wouldn't be beyond belief that Vetinari might want to have secret talks with a powerful empire whose existence is denied by the nations of the Circle Sea while it is itself in denial about the rest of the world being anything other than a wasteland filled with vampire ghosts.
In the The Celebrated Discworld Almanak it is reported that a large dragon has come to Ecalpon and taken up residence, again. But then, telling tall tales of strange events in far away places is what Almanacs are all about...