BDC Castle Access requires collecting four different Spectre statues and completing a chain of NPC interactions to gain access to the Castle basement (Consort / AA area).
Requirements
Access to Barb Fields and BDC (Black Dragon Castle)
Ability to kill Spectres and LairBear
Walkthrough
Get Gloves from LairBear.
Climb up in Barb Fields and talk to the Trapper — this activates the Archaeologist in BDC.
Collect a set of 4 different Spectre statues from Spectres in BDC.
Bring the statues to the Archaeologist in BDC.
The Archaeologist grants access to the castle stairs leading down to the Consort / AA area.
Tips
The easiest approach is to kill the NE Spectre last — it's closest to the stairs.
Beware: the stairs hex is a super heal hex for the Spectre, making it much harder to kill there.