New answers tagged level-design
To build level geometry within Unity, probably the best tool available is ProBuilder: http://www.protoolsforunity3d.com/probuilder/ However, the usual approach is don't build your level geometry within Unity. Build the level geometry in a 3D modeling tool, like Maya or Blender, and import that into Unity.
What you try to do is practically impossible. The primitives are there just for prototyping/placeholders. You will have to "get" those from outside of Unity. Or you can look at asset store, there are some assets sold that can do limited 3d modeling.
Is there any reason you are using primitives over meshes? The engine shines if you import custom models. So you can avoid the tedious process of modeling in unity by modeling in a program that is actually designed for that. The only other way I know off creating something like a door in unity without using multiple primitives is coding a mesh for it by ...
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