Greenhaven castle is a Greencloak-fortress and the Greencloaks command base the castle is very hard to penetrate and has many guards it has only been penetrated once before in Broken ground by Zerif and his army.(Except for Shane)
In the first series this is were the four fallen and their human companions operate from. The greencloaks guards the talismans here.
Drina had tried to penetrate Greenhaven and steal the talismans guarded there but she'd failed multiple times which was one of the reasons causing to her death.