Ecstasy World called.
The Radiant Ruins are a bit of a change in tone from most of the areas traversed to reach it. It's a large, open landscape of many different concrete structures towering over a vibrant, kaleidoscopic, marshy surface, speckled with ferns and odd looking plants, and accompanied by a relatively spirited background track. The place is uninhabited save for one small, hard to see silhouette, who wanders underneath a large, overpass-like structure. This little fellow does naught but turn translucent upon equipping the Glasses effect and dropping a guaranteed 50夢 when eviscerated by the Chainsaw.
On the west side of this world (south-southwest of the entrance) there is a bench and a red Vending Machine that sells sodas. Conversely, on the far east side of this world (just northeast of the overgrown doorway), there is a large plant growing at the side of one of the buildings. Interacting with the large leaf closest to the ground will let Urotsuki sit on it.
To the northeast of the main entrance is an overgrown doorway inscribed with ornate patterns and a depiction odd two-faced creature. Equipping the Bunny Ears effect near this doorway will grotesquely deform the odd creature's face.
The overgrown doorway leads to a dark, pinwheel-flanked bridge of sorts. At the end of the bridge is a peculiar-looking metal doorway adorned with eye symbols, leading into a large dark building, but it's currently blocked by a large metal barricade, inscribed with the characters 工事中 (under construction).
Periodically, the background track will fade out and a brief, frantic tone will play, along with a change to the color of the ground. This can happen once every thirty seconds, but usually takes longer.
As of .100d patch 2, the pinwheel bridge now has an detour on the left that leads to a staircase that loops around that the large metal door and take the you to a series of pathways connected by stairs that has 2 doors. The bottom door is currently blocked off while, the other takes you to the Bottom Garden.