You access this via the guy inside the 'The Twelve' towers. Once you get them there they'll have an automatic GH caste on them. There is no minimum level to access this L18+ area and you'll have an extra +4 morale bonus to your haggle skills.
They have armor and weapon pawn brokers next to each other so can sell you high end loot for a lot more and access to your bank, AH and other vendors. They are all in one area which makes it easier then running from House D to House K to sell stuff at pawn brokers.
You might even find some nice stuff for sale there too.
