Wel,
Well, in the original chapter, the player controls Renault Van Durce and Holy Bastion hero, while Marin remained inside the city, beside the weapon. If the map becomes more difficult in terms of stronger enemy waves, we may gain control of extra heroes, I supposeAccording to my 'investigations', the rocks and the pond in the background is indeed from "Lords of the Golden Guard" so you're right. There are a few Farms just out of sight in the top right corner, so the Golden Guard's outer defense line is also still there.
I can't imagine all of these Heroes being playable though. Maybe they're all controlled by allied players, but since they look like they're representing their respective unit-types, maybe each attack wave will have one of them as a leader?
If the allies indeed get stronger, I'll try to win without Marin's Airships.