okay, this is a tough question to answer b/c it requires so much explaining, and it seems you really know nothing about the game yet, so it is hard to begin. I will try to give some answers, but you will probably still be confused, but in a few months, will not believe you didn't know anything : )
First, I will assume you are not on a co-op server (Mordred).
There is an area, called New Frontiers, where all three realms may fight each other. This area has three large land masses, that are indeed Hibernia, Albion and Midgard. Now you can only fight in these area, and can not enter the PvE sections of the land (thus, you do not have to worry about mids coming into Camelot and killing you, that is not the NF section of the game)
You can get to NF from many portals. One inside Camelot (probably the one you are talking about when you asked where those people are being ported to) can take you to the Trials of Atlantis areas, housing, the Shrouded Isles area, or New Frontiers (for this location, you would select Forest Sauvage).
new Frontiers has 21 keeps (7 per realm's mainlaind). Each keep has 4 towers near it also, but there is plenty of geography that does not involve these keeps and towers. This is the "end-game" of the normal servers. I would not advise you going there until you are level 50. You can type /rw and see the New Frontiers maps with the keeps and towers in it. If you are actually in NF, then the keeps and towers shows cross-swords to show where battles are or flames to show keeps and towers that might be under siege.
The game is very complex, and I can not begin to explain it all, but hope that helps a bit.