The Planes of Existence

Overview
While The Walled City setting doesn't quite exist within the standard DnD setting, it does share many of the same rules when it comes to other planes and the standard planar map is roughly accurate. You can learn more about the standard setting here.

Similarities and Differences
The world that most things happen on is still the material plane. Intersecting and overlaid on this is the Feywild or Plane of Faerie, a chaotic tangle of light and dark that is filled with fae creatures. Rather than there being a Shadowfell, this is considered to be part of the Feywild in places that have been drained of energy in various cataclysms. There is no Ethereal Plane to act as a transition between the Material Plane and the Inner Planes - travellers must instead journey through the Feywild.

The Inner Planes or Elemental Planes remain as standard, surrounding and enfolding the Material Plane.

Beyond the Elemental Planes is the Astral Plane, the realm of thought and dream that is the unshaped matter every other Outer Plane is formed from. Because the Outer Planes are formed of the same stuff as the Astral Plane, to some people the term is interchangeable. The main diversion from the standard setting is here.

In the Outer Planes, rather than a clear division into different Outer Planes for different alignments, there is instead an continuous sphere of godly domains that contains every celestial and fiendish creature, in different warring pockets. Rather than being organised based on alignment, the different areas of influence are organised by pantheon, or sometimes seemingly at random. More information about how this works can be found here.

The outer domes of the Negative and Positive Plane probably exist, and there are definitely Demiplanes. While no-one has been to the Far Realms, there are certainly signs of the presence of Old Ones in the world of the Material Plane.