Question: I'm trying to plan the pieces I'll need for a castle door.
I know I need 2 of [these doors](https://www.bricklink.com/v2/catalog/catalogitem.page?P=2554&idColor=11#T=C&C=11).
Based on the part list for [Adventurers Tomb set](https://www.bricklink.com/v2/catalog/catalogitem.page?S=2996-1#T=S&O=%7B%22st%22:%224%22,%22ss%22:%22US%22,%22rpp%22:%22100%22,%22iconly%22:0%7D), it looks like I'll need several [shutter holders](https://www.bricklink.com/v2/catalog/catalogitem.page?P=3581&idColor=9#T=C&C=9), right? Is there a way to fill in the curves around the top so the front is solid?

Answer: 
The [Adventures Tomb 2996](https://brickset.com/sets/2996-1/Adventurers-Tomb) set you mentioned uses [shutter holders](https://rebrickable.com/parts/3581/brick-special-1-x-1-x-2-with-shutter-holder/) to mount the door and a [1x6x1 arch](https://rebrickable.com/parts/3455/brick-arch-1-x-6/) to cover the space behind the doors:

[![door attachement](https://i.stack.imgur.com/dUlVj.png)](https://i.stack.imgur.com/dUlVj.png)

[![door assembly](https://i.stack.imgur.com/pMgLl.png)](https://i.stack.imgur.com/pMgLl.png)

It is also very common to use a [1x6x2 arch](https://rebrickable.com/parts/3307/brick-arch-1-x-6-x-2-thick-top-with-reinforced-underside/), as was done in [Dark Fortress Landing 8802](https://brickset.com/sets/8802-1/Dark-Fortress-Landing):

[![Dark Fortress Landing door](https://i.stack.imgur.com/aZCrt.jpg)](https://i.stack.imgur.com/aZCrt.jpg)

As I mentioned in [another answer](https://bricks.stackexchange.com/questions/6265/how-do-you-eliminate-the-gap-between-castle-doors), 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.
I have those doors, and you're right about the shutter holder bricks, which are pretty fragile. The doors each cover half the space under a 1x6x2 arch. Besides the shutter snaps, the only connection point is the stud/doorknob.