The Okolnir announcement (Asprika and Brynhild) was archived here:
WoE Update Part 1: Vision of OkolnirThe Halls of Abyss announcement (1.0 guild dungeon floors 2 & 3) was archived here:
WoE Update Part 2: Halls of the AbyssThey were both added as a means of providing additional rewards for guilds who invest a high level of economy in their castles.
2.0 castles reward opportunities at Asprika and Brynhild.
1.0 castles open a dungeon with the opportunity to fight exclusive monsters, chances at Vellum weapons, siege potions, and certain other equipment. They also allow players from other guilds to potentially enter the first two levels of the guild dungeon for a fee (but never the third level, the Corridor of the Abyss) depending on the castle economy, but guild vs guild has been enabled in the dungeon. We don't have them yet, but those mini-bosses and MVPs also have their own cards.
