How should castle doors be attached?
The Adventures Tomb 2996 set you mentioned uses shutter holders to mount the door and a 1x6x1 arch to cover the space behind the doors:





It is also very common to use a 1x6x2 arch, as was done in Dark Fortress Landing 8802:



As I mentioned in another answer, these clips generally attach to the back of the door, not the side. The geometry is more complicated to completely fill if you want to have the doors hinge from the side.