The Observatory

A towering, circular structure with a large, domed roof, the Observatory is perched atop the highest point of the citadel. Its walls are lined with countless windows, offering panoramic views of the surrounding landscape.

Central Observation Deck: A vast, open space at the heart of the observatory, featuring a colossal telescope pointed towards the sky. Ornate astrolabes and celestial charts adorn the walls.   Magical Lenses: Along the walls, a series of magical lenses offer different views of the world: some magnify distant objects, while others reveal hidden energies or beings.   Time-Based Anomalies: Due to the citadel's time loop, the observatory's view of the sky is often distorted. Stars might appear in unusual positions, or constellations might shift and change shape.  

Key Prism

The observatory's magical lenses are not merely for observation but are integral to unlocking the chamber containing the Key Prism. Each lens is attuned to a specific celestial body, and their alignment is crucial.   Every loop, 1 lens can be moved. After one has moved, they all lock into place. The lenses will not reset at the start of every loop.   Every lens has a specific celestial body etched into it. Next to the array of lenses is a lever, once pulled, it will shine a light through the lenses onto the a wall. If the pattern matches, it will open to reveal the Key Prism.   Once the hidden compartment is unlocked in a loop, it will stay open in the next loops.  

Potential clues

In one of the cabinets or drawers, the solution may be written.
An NPC might know the combination
It may be the celestial bodies they see through the grand telescope