what opened the area that exposes the decision chamber below the Citadel Tower?
Its possible that when Shepard opened the arms to allow the Crucible to be docked, it caused the decision chamber to be exposed and wasn't seen since it may of opened when the cutscene switches to showing the Crucible docking with the Citadel
Its possible that when the Crucible is docked, that it powered up that area to expose the decision chamber just before the beam is turned on.
Another possiblity is that the catalyst itself exposed the decision chamber before the beam turned on in the link above.
Obviously whoever designed the plans had to know the ins and outs of the Citadel for the crucible to fit perfectly with the Citadel and to have it work like it did. Here's a post that I believe who designed the plans. Its only speculation. It came from this thread which is the reason why I created this thread since I don't recall seeing a thread asking about how the decision chamber is exposed before the Crucible docks






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