Important Enemy Locations

These locations are the Vex Citadel, the Hive Hellmouth, and the Cabal Exclusion Zone. Of course, there are other locations we know of such as Charlemagne's Vault, but I'd like to focus on these. These locations are the central "bases" or "headquarters" of the various enemy races, and are heavily guarded. What kind of loot is found in these places? Another question is: what is the Fallen's central location, and where is it (if they have one)? If they do have one, I believe it's on Europa. I say that because Europa is essentially confirmed as a location, and early concept art of the Fallen features them in an icy environment. Of course the concept art could actually be showing the Fallen somewhere on Earth, but I still think they will have some sort of large facility on Europa. What are your thoughts?