Citadel
Quest: Citadel Contruction
Who: Architect
Where: Otherworld Capital (accessible in the Cave of War mages using the Portal to Otherworld)
Process: 3 stages
Getting recipe building "Oracle", "Warehouse", "Town Hall"
Stage I
1. 15 clan members with at least 7 secret knowledge need to register by talking to the Architect.
2. Bring 50pcs of each primary resource (Cobblestone, Log and Iron Ore) to the Architect. (Guide to the Clan Resources buildings in estate)
3. The clan leader should go to the Elder in the city square and ask him about the Architect's sketches of the Otherworld.
4. Obtain a total of 9 sketches in the following ways:
I. Obtain 10 Merchant Goods while hunting monsters, whose level is no more than 1 below yours. Return to the elder to receive an Archive Chest, within which you can find a drawing with a little bit of luck - meaning it is NOT guaranteed that you will get a sketch for an opening an Archive Chest. (Note: if you have 10 Goods in your backpack, you will not be able to receive more until you handed them in to the elder.)
The clan needs to collect 8 drawings, which if obtained directly go into the clan leaders backpack. (Barber's sketch, Arena Sketch, Temple Sketch, Bank Sketch, City Gates Sketch, City Wall Sketch, Guild of Artisans Sketch, City Square Sketch)
II. For the victory on one of the battlefields (Arena, Crystalline Caves or Temple of the Chosen) obtain the lost archival draft - obtaining the drawing again is random, so it may require several wins in battlefields. If the drawing drops after a win, it will also go straight to the clan leaders backpack. (Registrar Office Sketch)
The clan leader needs to bring them all to the Architect so the clan receives access to the clan citadel and the Oracle Sketch (which allows the clan to build Oracle I in their citadel if they have 30 Boards, Construction Stones and Sheets of Iron)
Stage II
Using the Elemental Cube call upon the elements of the Underworld to fight. The clan needs to kill 500 Elementals combined. Every member of the clan has an Elemental Cube in their backpack, which they can only use in the Otherword Capital.
Once using it you will be attacked by one Elemental, though using the cube from your combat backpack allows you to summon an arbitrary amount of Elementals against you in a fight. You can NOT summon mounts or other summons and no one can join to help you in the fight - thus consider how many Elementals you want to summon. The Eelementals strength depends on the players level, they do not drop money or loot, but also do not give experience.
The count of how many Elementals have been killed can be viewed in the clan leaders profile.
For completing the task you will get the Warehouse Sketch (allowing the clan to build the Warehouse I in their citadel, requiring 250 Clan Energy, and 50 Boards, Construction Stones and Sheets of Iron).
Stage III
1. The clan leader should visit the sorcerer at Barrow of Sadness or Gorge of Gondi.
2. Clan members have to bring to the sorcerer 6000 pcs of quicksilver. Quicksilver can be handed in by any clan member in amounts of 100, 500 or 1000 pcs., the progress can be monitored by the clan leader in current quests.
3. The clan leader needs to bring 100g to the sorcerer.
For completing those tasks you get Townhall Sketch (allowing the clan to build the Townhall I in their citadel, consuming 1000 Clan Energy, 500 Boards, Construction Stones and Sheets of Iron)
Construction
For the construction of any building in the Citadel the clan must have the appropriate sketch. The first three buildings must be placed in the correct order, which is Oracle first, Warehouse second and then the Townhall can be built. Further buildings and upgrades may also require others to be built or upgraded first. More sketches can be bought in the shop you can access when talking to the Architect in the Otherworld Capital for Stone Architect Signs. Those can be acquired by completing the clan collection "Stone Architects Artifacts".
Clan Energy can be obtained by doing Oracle quests, how it all works is explained in the Oracle subpage.