It's build like a maze deeper and deeper to the heart of the planet. To the new world.
After the last attack from Moloch you are able to enter the finished stairs to Molochs empire.
He declares the Skull of Doom complete and that he can come and go as he pleases then accidentally says that you can as well. You then get the opportunity to follow him back, and destroy his bases as you make your way down to the centre of the world.
Outposts of Moloch wait for you to be conquered one after another.
Once You reached his headquarter Moloch says:
"You have reached my headquarters, but this is not where your intrusion will end. Behold the Power of INFERNO!"
Have fun discovering what comes next.
After destroying Molochs last base,you'll be able to build your underground-backyard(An Inferno Yard).
There will also be a map room like the map room level 1 upon the surface. May the new ages begin...!
TIPS: The Compound/Habitat (where the Monsters are stored) also serves as a Monster Bunker. As you get lower, the monsters change in accordance to "strength". Loot accumulates as you destroy yards for the end, when you destroy Molochs last yard.
Tip: If you recieve a gift from a friend you can choose to recieve it in the regular base, or if you open it in the underground base, you can recieve one of the underground resources.