Kaineng City/Center isn't the "Marketplace"
They are two different towns. Marketplace is south of Kaineng City.
If a Canthan Character travels to Tyria, s/he will land on Lion's Arch. According to the teaser quest for Factions, the Canthan's do not know what's happening in Tyria and they think that the White Mantle are the good guys in Tyria.
Additionally, with the presence of the Ascalon settlement, their knowledge of Ascalon is limited to the Ascalon refugees. Thus to their knowledge Ascalon was conquered by the Charr and is no more, and thus they won't care "normally" about the Eastern side of Tyria.
But as Canthan characters do the missions, until they reach the Titan quests, they will learn that Ascalon still exists and the King is trying to defend Ascalon from the invading Titan Lords.
Now, let's forget about RP/SL (SL = storyline). Let's talk about technicalities.
Once your Canthan character is at LA, you can trackback all maps, quests and missions up to the Ascalon city. No pre-searing, coz pre-searing happened 2 years before the current timeline (speaking of that, Shiro was killed 200 years before the current timeline, I wonder what's with the number "2").
Start the missions and quests from Ascalon city, though you'll be able to solo all of it since those are low-level missions and quests.
Its the same with Ascalonian Characters going to Cantha continent. They will land on Kaineng City, the continent's port. They can either continue on until the final mission and quest SL related but they are not restricted from trackbacking the rest of the region until you reach the Monastery - which is Cantha's training grounds. The question tho, can we access the Monastery if we will come from the Cantha mainland (the Monastery is a set of islands South-West of the Cantha continent mainland).
Now RP/SL related:
Ascalonian characters were called by the Kurizicks and Luxons to help them in their wars. They landed on Kaineng City and instead learned there's a deeper trouble. They are asked to travel to Marketplace and meet with Mhenlo and his former master (who I forgot the name) and they're asked to unite the Kurzick and Luxons.
That's basically it