Known Locations
All this information was pulled from the website lexicanum and some others
Terra
Terra, also dubbed Holy Terra and known in antiquity as Earth, is the homeworld of Mankind, the resting place of the Emperor, and the most holy and revered place in the Imperium. Pilgrims throughout the Imperium flock to Terra - even the barren soil that pilgrims tread upon is considered holy. It is effectively a temple the size of a planet.
Mercury
Mercury is an Imperial Mining World that lies near Terra in the Sol System.
Imperial Palace
The Imperial Palace is a vast edifice that serves as both the palace of the Emperor of Mankind and the capital city of the Imperium of Man. Built across a large part of the Asian continent on Terra, the Palace is divided into the Inner and Outer Palaces. The Palace is built to truly monumental proportions: seen from orbit, it consists of connected kilometres-high monolithic and pyramidal structures, marked with landing pads and studded with defence weaponry. The palace itself is the seat of the Adeptus Terra, but is overseen by the Adeptus Custodes.
Not only is the area of the Imperial Palace enormous, its macro-habs and space ports break through the atmosphere and rise into the void, and its sub-levels dig deep into Terra's holy bedrock, in some places reaching a depth of hundreds of miles below the surface. Its vaults, corridors, fastnesses, plazas, and chambers are so multitudinous that no single record remains to list them all, and the techno-urbanic serf tribes, clan holdings, and societal sub-nations that dwell within its walls could populate entire star systems.
Construction of the Imperial Palace began after the Unification Wars as a symbol of the Emperor's might and a monument dedicated to mankind's new unification. Based around the Himalayan Mountains, the Palace was designed by the most expert artisans of the many Masonic Guilds of Terra. The early management of the Palace was left to the Seneschals of the Departmento Regia Interior.[29] Construction of the Palace defenses was overseen by Primarch Rogal Dorn, much to the frustration of Perturabo. Some construction of the Palace was still taking place by the end of the Great Crusade, meaning construction took at least 200 years.
Inner Garden
Within the confines of the Outer Palace were the legendary Inner Gardens. In this garden was erected very high walks, supported by stone pillars; the Inner gardens were a pensile paradise and were constantly replenished with all sorts of flora and fauna drawn from across Terra and the human-settled galaxy. The Inner Gardens sloped like a hillside and several parts of the structure rose from one another, tier upon tier, the appearance of the whole resembling that of a theatre. Unfortunately, during the Siege of Terra when the forces of the Warmaster Horus breached the walls of the Inner Palace, the battle raged across the grounds of the Inner Garden, turning the vast parkland into a charnel house. It is unknown whether or not this beautiful edifice was rebuilt following the end of the Horus Heresy.
Malcador's Tower
Malcador's Tower, also known as the Tower of the Sigillite, was a key structure located within the mountainous Himalayasia region of Terra, serving as the headquarters for Malcador the Sigillite. It was absorbed into the vast Imperial Palace, serving as a bastion of knowledge and lore management during the Great Crusade and Horus Heresy, with its top often depicted as featuring a floating representation of the Solar System held by anti-gravity technology.
Dark Cells
The Dark Cells or Black Cells, are oubliettes deep beneath the Imperial Palace on Terra. These cells contain entities and artefacts of Old Night that could annihilate the Imperium if they were ever to break free. The newly emerged threats of the galaxy that must be imprisoned therein are also added to the Dark Cells. Part of a complex of 81 levels, they are guarded by a Shield Host of Adeptus Custodes known as the Shadowkeepers; their security includes rune locks, wards, and sanctic circles throughout corridors patrolled by a hundred Custodians at all times. Although neither light nor sound can escape the cells, the air of dread puts even Custodians on edge. Not all of the prisoners are truly alive, but all have anima.
Garden of Nurgle
The Garden of Nurgle is a realm within the Warp that falls under the dominion of Nurgle, the Master of Pestilence and one of the Chaos Gods.
This unwholesome realm is home to every pox and affliction imaginable and is alive with the stench of rot. This 'garden' is not a barren wasteland, but rather a macabre paradise of death and pestilence. A thick sheet of buzzing swarms of black, furry flies litter the sky, and twisted, rotten boughs entangled with grasping vines cover the mouldering ground, beneath an insect-ravaged canopy of leaves. Boughs of Gnarlwood host the dormant Daemon waiting to be reborn. Defiled fungi both plain and extraordinary break through the leaf-strewn mulch of the forest floor, puffing out vile clouds of spores. Muddy rivers slither across the bloated landscape. Nurgle's Blighted Mansion of Misery and Mirth, made of rotted timbers and broken walls, resides at the heart of the garden — decrepit and ancient, yet eternally strong at its foundations. It is within these tumbling walls that Nurgle toils at his cauldron, a receptacle vast enough to contain all the oceans of the worlds of the galaxy.
Fortress of Khorne
The Fortress of Khorne is the domain of Khorne within the Warp.
On the outskirts of the Fortress of Khorne are a ring of volcanoes known as Khorne's Rage intersected with rivers of boiling blood. At the base of these volcanoes lie the Daemonic Forges of Khorne, which churn out his endless Daemon Engines. Within the same area are the Bloodpits, from which new Daemons of Khorne arise.
Crystal Labyrinth
The Crystal Labyrinth is a maze within the Warp that is spread across the domain of the Chaos god Tzeentch and its presence is felt by all within the Daemon realms.
The Crystal Labyrinth sits upon an immense iridescent plateau and consists of evershifting crystal avenues and glitter in every colour. Within the labyrinth sit hidden pathways that are forged from lies and deceit which link to domains of the other Chaos gods thus linking the fractious Realms of Chaos. At the very heart of the maze of corridors sits the hidden and majestic Impossible Fortress and Hidden Library.
Palace of Slaanesh
The Palace of Slaanesh is Slaanesh's realm within the Warp.
Those that dare his realm risk becoming trapped in its warped delights for eternity. The Dark Prince's realm is divided into six domains, arranged in concentric rings about the Palace of Pleasure. The palace itself can only be reached by passing through all six domains.
The Great Green
The Great Green is the spiritual fungoid realm, where the Orkoid species' Gods Gork and Mork reside and it is also where the Greenskins' spirits go after they die.
There their spirits will reside, until Gork and Mork decide to reincarnate the deceased Orkoids into new bodies. These will correspond to whatever subspecies the spirits belong to. As the Prophet of Gork and Mork, however, the Warlord Ghazghkull Thraka seeks to carry out their will, by making the Great Green a physical realm as well. In order to do this, he intends to unite the entire Ork species under his command and then conquer the Galaxy, in their Gods' names.