Astral Clocktower is a location in Bloodborne. Its located at the top levels of the Research Hall.
This lantern unlocks when you kill Lady Maria of the Astral Clocktower.
After unlocking the door using the Astral Clocktower Key, walk towards the center of the room to trigger a cinematic. What follows is a fight with the lady herself. For a fight walkthrough, click here.
Defeating Maria will grant you a buttload of Blood Echoes, a lamp in this boss room, and the Celestial Dial on the floor. Approach the giant window at the end of the room and a message reading "Hold up Celestial Dial" will prompt. Doing so will create an circular opening in the window ahead. Proceed through to enter the next area: Fishing Hamlet.
(Work in progress!)