To reach the Tower of Kagutsuchi, you must head to The Obelisk at the Assembly of Nihilo. Once you get here, you want to take the elevator to floor 140. From there, you should be able to find the nearby lift that will take you to the 142nd floor. There, you will find a light that will let you reach floor 150. At this point, you will find the Tower of Kagutsuchi that you can now enter.