24. What was the Citadel apart from a massive space station?
From Quiz "Mass Effect" - Universe [5]
Answer:
A Mass Relay
The Citadel was a large space station at a busy node in the mass relay network. This made it very easy for species to find it, and to branch out to the rest of the galaxy. The galaxy's space faring civilizations believed it to be the pinnacle of Prothean engineering, along with the mass relays. It was the heart of the Prothean empire.
This was only half true. The Protheans found the Citadel the same way the modern species found it, as did the species that came before the Protheans, and so on. Every 50 000 years at least one species would achieve spaceflight, and would find the Citadel. It would become the hub of their empires that would link all their colonies. And then they would disappear. The extinction events were so complete that humanity's colleagues in space only had solid evidence of the most recent extinction.
It turned out that the Citadel and the relay network were not constructed by the Protheans, as originally believed, but by their exterminators, the Reapers. The Reapers built the network so that any species that found it would develop their technology along paths that the Reapers desired. The space-faring species would make their way to the Citadel, the heart of the network, and set up a galactic government. By accessing the stored data on the Citadel, the Reapers could easily learn everything there was to know about a species, including the location of all their colonies. The Reapers could then quickly spread out into the galaxy from that central node.
It was eventually revealed that the Citadel itself was a giant mass relay. It connected to dark space, where the Reapers waited, and it was their gateway into the Milky Way. The Reapers were so advanced and massive that they could easily overwhelm any security guarding the Citadel when they suddenly entered, and would be able to download the Citadel's data stores before its occupants could ever figure out what was going on.
This cycle of extinctions was put on hold by the Protheans. They knew that the Citadel was maintained by the Keepers, and they they would open the Citadel for the Reapers when a certain signal came. The Protheans reprogrammed the system to ignore the signal the next time it was sent, buying future races time. When the signal was sent and the Reapers did not come, Sovereign, a Sentinel of the Reapers left in the galaxy, set about fixing the situation.