you have to enter via a quest with Rafforty called "An Ice Merchant's Dream". it asks you to collect silver hemocytes; everyone in your party must have the quest engaged, and needs to have at least 10 in their inventory to enter. the castle itself is an instanced hunt with a boss mob at the end of it, so you can only go in with one party of people.
What are these silver hemocytes? I have lots of silver ice crystals, but I cant get in with them. I know you can try to change them into black ice crystals, but I have failed every time with that.
on your quest list for "an ice merchant's dream" it should show one drop down item, which says something about the ice crystals. when the second part of the quest has been activated, there will be a second drop down item saying "save the silver hemocytes".
the only members of our alliance that have this part activated also completed the other quest from rafforty, so it might be loosely tied to that one as well. sorry i dont really have much more info than that.