On Kain The Hero Monument says that some of the Heros have partaken in the raiding of her 2 or more times...so it would seem to be important/benificial to raid her for more then just the circlet.
Wel, this i know.
(the patch notes doesnt say squat btw).
You need 10 silver hemocytes to enter.
Shes lvl 60
You can only go with one party.
You have 35 minutes.
She has a healer and minions.
And its mobs inside the castle as wel.
You get the circlet after defeating sirra (and some mobs close to the ice queen?) by talking to the ice queen.
The ice castle is an instance.
The circlet is blue :P