The Overgrown City resembles just that, a city in which some disaster occurred, that has long since been abandoned and mostly reclaimed by nature. The city is devoid of life, except for a few non-NPC creatures that are only debatably alive at all. The City can be divided into three rough sections. For unknown reasons, you are incapable of using the Bat Effect while in this area.
The City can be reached from the Industrial Maze's Garden area, via a door just above the garden itself. After traversing a short passage fraught with amber light, enormous plants, and a strange, mechanical being, you will exit into what appears to have once been someone's back yard, via a manhole. Following the linear path from here will take you into the main part of the Overgrown City itself.
Main Area Edit
This area is extremely tall, and quite wide as well (though going too far in either direction will smack you with a dead end in the form of traffic cones). Throughout the area you will occasionally find what look to be enormous lotus petals, these vaguely resemble Steve "Leif" Kareha, one of the most obscure NPCs from the original Yume Nikki.
You begin at the top of the area. Going down a little bit, and you will see many lampposts lining a path. There is a secret entrance which is too dark to see unless you use Rainbow effect between two lampposts. Enter there and you will see a statue consisted of old gold. Note that entering hidden entrance makes it unlock BGM 078 D, so if you want to fully unlock BGM 078, it may important. Back to the main area, as you travel down relatively linearly, eventually the BGM will change to a loud, melancholy piano riff, and you will be in "the bottom" of the city proper. There is a black cat-like creature here that will follow you around if you have the Wolf effect on. Going east from here, you will encounter another garden not dissimilar to the one found in the Industrial Maze, here, you will find a strange creature composed of orange rock and traffic cones cowing under a disused overpass bridge, possibly dead (although it is still possible to kill it with the Chainsaw). Going further east and through a narrow path along the towers, you will run into a massive creature with what appears to be a ball of scrap iron for a head, bound up in what looks like an enormous straitjacket. If we allow ourselves a moment of whimsy, perhaps this is what caused the disaster here in the first place.
Also, in the "Overgrown" part of this map you can found a doorway that led you to a white area. Somewhere in this area you can find another doorway that takes you to a strange subject. Going to the left you can reach the Chaotic Buildings.
Going yet further east will cause the BGM to change again, and put you on the path to the Blue Room.
The Aquarium Edit
This building is found on the easternmost edge of the map. Currently, there is very little here but a dark blue glass wall. What purpose this may serve eventually is unknown. A flight of stairs located right in the center of the Aquarium would lead Urotsuki into the Victorian Drains.
Nexus → Red Streetlight World → Gray Road → Grass World → Green Neon World → Construction Frame Building → Cog Maze → Forest Pier → Abandoned Factory → Arc de Pillar World → Flooded Ramparts → Mansion → Industrial Maze → Overgrown City