Some time after the Third Overlord disappeared, before the Cataclysm destroyed the Dark Tower, Gnarl and the minions fled into the Netherworld to wait for a new Master. When the Dark Tower was destroyed, the minions took residence there permanently. When the Overlord first arrives, the Netherworld Tower is only a mess of rock caverns but progressing trough the game will make it cleaner and will become a true palace worthy of a Overlord
The primary chamber of the Netherworld Tower. It contains the stairway to the Private Quarters, as well as platforms on a balcony leading to the Foundations, and the Minion Burrows. The primary feature of the Throne Room, is the throne itself, which allows you to teleport to various locations, hold a peasant audience, as well as access your quests.
This is where every thing you collect, aquire or conquer is kept.
Here is where all your gold is kept and where all of the the romantic action is.
Tower Upgrade Room
Here you can update the architecture of the tower. The interior decorations you can choose from depend on whoever your current First Mistress is. The set of mistress decorations does not change automatically after switching to another First Mistress, however, it stays the same, until you (re-)buy the new deco. For a complete list of available interior decoration Netherworld Tower Upgrades, see the taste of the appropriate mistress: Kelda, Juno or Dark/Ghost Fay. The Overlord does not have much say in decoration, other than pay for them, he does have a banner though.
This is where you may choose, who you want your First Mistress to be. This has an influence on what Tower Upgrades become available for purchase, and also determines what mounts become available for your minions in the final battle against the empire.
This is where the minions live,and you can access the Graveyard to resurrect your favourite minions that have died in battle, but you will have to sacrifice quite a few (i.e. 2 x <Level of Minion to Resurrect>) minions of the same colour to bring back one more powerful one. You may also access the Barracks of the different minion tribes where you can view your minions that are all individually named and each of them have their respective titles based on their actions,see how many minions you have spawned\died\alive.
Battle Rock Arena (DLC)
it serves the same function of the Dungeon of the Dark Tower in Overlord,it's a place where you can fight waves of enemies,and some that aren't present in the story campaign like the Phoenix and the Satyrs(though their character files are still in the vanilla version of Overlord 2,as the Arena itself)
There are many objects that can be brought back to the tower, see Netherworld Objects.