My 3D model is printing with missing parts when sliced in Ultimaker Cura I have been working hard the last year on the model below. I am new to everything 3D that is modeling and especially 3D printing. I have however successfully concluded quite a good number of 3D prints which I created in blender with my Creality Ender 3 Pro so I have a bit of experience.

All this new experience for me started with the desire to do this project I have been working on for all this year. A complex 3D model of a knight's tomb which I would like to print.



As you can see Cura is clearly indicating that there is a need for supports in these red areas. The model will be printed in a 15&nbsp;cm size. I have also managed to resize the model from a 22 million face mesh full of holes to a 900k manifold model.

However, when I slice it I get this.



As you can see supports are only generated for the outer column part. None are generated for the arches which are totally absent when the model is sliced.

I have tried to alter the model's orientation but with no result. I will try to separate this mesh in parts but it would mean restarting all from scratch since I found no good software to slice it precisely.

I am quite sure that the main problem lies in the fact the Ender 3 pro is an FDM printer an that the vertical lines of the arches are too thin.   Since I tried to upload my model on 3D printing services to see if it could be printed in other materials and could be printed in finely detailed resin.  

I would like at least to know if I am right in my problem spotting or if there could be any solution to the present problem so that not to go wrong if I will redesign this part.
There's a setting in Cura to Print Thin Walls. It's turned off by default, and IIRC correctly there's a good reason it's off by default. But for this item, you should turn it on. 



Update

Looking again, I see this is vertical, so I'm not sure how well this setting will help. Even if it does create the gcode to include the walls, this would be a tough print. If possible, you may want to rotate this 90° for printing, though I also see the gap on the right that would become a long bridge.