How can I get the level to start in area 3?
I tried setting this:

To something which I thought would make it overflow into the next area.
(aka area overflow bug).
But it didn't work.
Maybe I typed in the wrong value?

Any ideas?

Or it's hardcoded...
I could look at the code to check, I saw some code that handled starting entrances back then. But I have no idea.

I dont think its hardcoded. My contest level is to appear to enter in area 3 in the midpoint, and I originally did the level over 1-1. I imported 1-1 onto the other 2 areas.
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I am pretty sure too that it is not hardcoded. I and ray managed to be spawned in different areas by just swapping NMLs around, IIRC.

So when I enter the 1-castle from the world map, with 1-1 area 1 imported into area 3 after the black screen displaying "World 1 Castle" appears he will appear in area 3 midway point? nomatter what area 1 details are? nomatter area 3 start point details are?

EDIT:
I am asking! Not confirming.

Yes, I think it will work that way. I have many problems in my hack due to the midway points, and this is the cause why

It seems the game takes priority to the last area, since if you import a level that has to appear there in midway point on all 3 areas, it will appear at the last area of the level.
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It did not work but I am not sure I set it up it correctly.

I've decided I don't need it to start in Area 3 anymore.

No, not exactly.

I wanted the main castle room and boss room to be in the same area with no warp/door/pipe transition.

But I think the memory was overloaded with the graphics of the main level and boss textures combined.