Help how can I draw an area in scene A to display RPG battle content?

I’m a newbie and need help from an expert. I want to create a battle area in this scene, the part with a red frame, and then do some RPG battles in the frame. But after two days, I still don’t know how to implement it. Is there any expert who can give me some guidance? Thank you.

Now only this scene has been completed, and there are jumps, but the battle areas have not yet been implemented.