Endless Tower is an instanced map (Like Orc Memorial Dungeon and Sealed Shrine) that it will be only created one a party starts the instance.
It consists of 101 floors, all of them containing different monsters. Some of them contain strong MVPs like Ifrit, Fallen Bishop or even Valkyrie Randgris.
Once you manage to pass all the floors you'll be able to fight Eintweihen and Naght Sieger at floor 100 and 101 respectively.
It's indeed a challenge.
The map/dungeon are indeed there.
The instance it's just the one that it's not implemented, therefore you can't go to ET.