Carving openings in a terrain for already built subterrain structures

Question

I built a structure that is mostly underground but whose entrances (three) are partially above the surface.
.

I can duplicate the vertexes and edges where I want the openings on the terrain, but then I have to create the terrain and to sculpt it or modify someway to create something similar to this image.
and the front view .

The problem I have is that when I try to sculpt, the opening gets deformed as well. Also that happens when I use proportional editing. Is there something I missing, or just a simpler way to do this in Blender?

You can hide the unwanted vertices in edit mode before sculpting, or use a mask brush — lemon, Aug 16 '16 at 17:48
It's not quite clear what's the problem. You'll need to model terrain either way if it is used as an entrance. If your problem is that you created a terrain and while sculpting it you notice that entrance is being sculpted too, then you have these meshes joined into one object, see http://blender.stackexchange.com/questions/41205/how-can-i-unjoin-meshes — Mr Zak, Aug 16 '16 at 20:16

score 1 · Answer 1 · answered Aug 27 '16 at 15:59

Sculpt your terrain the way you want it too look. At this point do not worry about covering the entrance to your bunker. Just get the height along the sides and top where you want it.

In this picture I have my terrain set to be partially transparent so you can see the bunker entrance I covered.
terrain

Add a cube, transform it so that it sits inside the walls of the bunker entrance. Make sure that the cube extends a bit below the low point of the terrain, and above the high point. In this image my new cube is selected in orange, notice the bunker walls around it on every side.
boolean object

Now select your terrain and add a boolean modifier. Set the Operation to Difference and set the Object to the cube you added in the last step.

Once you are sure that everything is set, you are not going to move the bunker entrance, and the cuter cube is in the right place, apply the boolean modifier. I always duplicate my object and move it to a different layer before applying modifiers like this

This is what it should look like after you apply the boolean modifier. The door will still be blocked off, but we will fix that next. (circled in red in the image below.)
boolean modifier applied

Last step, Tab into edit mode with the terrain selected. Select all the faces the boolean modifier created, Bottom sides, and any little faces on the terrain that are completely inside the entrance's walls. (you don't need any of the sides, because they are in the entrance object). When you have your selection done, press X or Delete then choose Faces.
terrain in edit mode with faces selected

Carving openings in a terrain for already built subterrain structures

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