Or alternativly, dare i say it, use easygen, as this buils the map as brushes, which you can then delete around where you want your bunker to go and then add extra brushes in to fill in any gaps.
That cliff looks great! I tried to make a cliff with meshes but you could see the joins in game, have I just got to overlap 'em or something?
Would you be able to send me that map file so I could see how you made it?
Yes, it is a VERY nice job.
Really, the only way to make those kinds of cliffs with meshes is to slightly overlap them.
It looks great because you will get a slight shadow where the meshes overlap, just as if it were a crack in the cliff face.
The trick is to make sure your overlap is not perfectly straight, and that you adjust mesh vertices to "texture" the cliff face so it too is not flat.