Green Zone is now known as Greenhaven:

Quest: https://drive.google...BLX2EB3YyhjeH-E
...and that guy will be the first to tell you.
He's essentially a gatekeeper who welcomes you and introduces you to the place you just broke in to. His dialogue strings give you the official breakdown:
194: Welcome to Greenhaven!
195: Our community took refuge in this abandoned labyrinth
196: when the lands above devolved into an endless battle ground.
197: You show skill and cunning to have passed our security measures.
198: You are welcome here.
199: We have tried to make our new home hospitable, but be cautious...
200: it was first, and always will be, a dungeon labyrinth.
Through the upper door should be their main stay, with all the shops, residences, and whatever else you think would suit an underground community. Keeping the overworld in mind, I'd expect the place to expand more horizontally than vertically, but I suppose we'll see.
The upper-level staircase is connected to the Whistle-blown staircase on the overworld screen above (Map 1 Screen 37). That particular screen has had its stair secret fixed (the combo was never assigned), and now triggers instantly by using Whistle trigger flags rather than Whistle -> Stairs screen data. This means the old man with his Heart Container Piece should probably be moved to wherever this new Greenhaven path ends. (Ideally, this path shouldn't be connected to the main level, or else it would negate the extended entry process so far.)
Music has been added. It was really hard finding something that covers both abandoned and inhabited, dungeon and town, safe but barely. It's possibly not the optimal selection, and a little redundant, but the NES Deja Vu title music is oddly close to what I had in mind, so it's in for now. Still open to suggestions though.
Torches have also been placed in previous rooms for better consistency, whether added in new positions or replacing old statues.
Finally, that door at the bottom is actually a shutter that opens when you unlock the door at the top of the screen below, but you'll basically never see it closed unless you find the Whistle stair secret first, so I screengrabbed it being open.