Here is an in-detail description of our headspace and everything in it to the best of our knowledge.
The City is a yet-unnamed coastal city that makes up the innerworld, or headspace. It doesn't closely resemble any outerworld city, but we believe it to be a combination of multiple major US cities, such as San Francisco, NYC, and Chicago. A few of our alters who don't front often can be seen going about their daily lives throughout the city, amongst several NPCs (those who are not alters and exist to "fill out" the innerworld). For example, our Jake Lockley has taken up a taxi service in the city, and our Grillby happens to run the café on the ground floor of the apartment building. Most of the city is unknown to most of the system, as the frequent fronters tend to not leave the apartment building.
The café makes up the ground floor of the apartment building. It is home to the café itself, and the fronting room. The café has several seats in which NPC guests are allowed to sit and order food, as well as apartment residents. There is a large beanbag chair in one corner, the same shade of green as the leather cushions on the seats and booths. The walls are done with vintage wood paneling, and the lighting is natural with large windows on the front and side of the café. Plants hang from the ceiling, as do the bare pendant light fixtures.
The fronting room is best described as a VR gaming room, complete with a headset and 360° screens. Deliberate fronting and co-fronting can be achieved by visualizing oneself in the fronting room, placing the headset on your head. This doesn't always work, but with practice, we seem to be getting better at it.
Discovery of our office is fairly new, though we've been interacting with the office and file cabinets since before our first system discovery.
The office is, in short, where we store our memories. It is on the first floor of the apartment building, across from Eren and Aster's apartments. There are various file cabinets in this office; some are locked, some aren't. In order to retrieve or "file away" a memory, we visualize ourselves thumbing through file after file until we find the appropriate section.
The office has the same wood paneling as the café, with one window behind the desk. The desk sits directly across from the door. On top of it sits a desktop computer and a desk fan. The walls are lined with file cabinets and paintings of nature, such as forests and rivers.
The apartments start on the first floor (above the ground floor). There are an unknown number of floors, but it's not infinite. Some headmates room together while others have their own apartments. Some have studio apartments, some have one bedroom, and some have two bedrooms. The floor plans are all varied and depend on the preferences of the respective alter(s).
The park is an almost exact replica of the one from our hometown. It consists of a playground, a forest, and a large open field. The main difference between the one in the innerworld and the one in the physical world is the size of the forest; the one in the innerworld is easily four times larger.