re: rubble
I find that it takes a MUCH greater volume of rubble than you would think to make piles large enough to be realistic.
To that end, cut up styrofoam insulation, or electronics packaging material and build up stages in thin layers into a pyramid shpae, but don't make then too regular.
Then paint them a dark brown.
Now add kitty litter to fill in and start covering the edges, white glue water in place.
I have not tried it, but it would seem that white glue and kittly litter are water soluable, so thinned red brown paint in there should start to color the litter as well.. um.. brick litter. I use the REALLY cheap sutff, not the more expensive clumping kind.
The top layer is then made of plaster or cork scored bricks, wooden coffee stirrer planks, and microscope slide cover glasss shards you can use the plastic 'bubble' from kids toy packaging, but it does not look as much like broken glass as real broken slide covers do. (Use LOTS of broken glass, but make the pieces small, this is 1/35th of life size after all) plumbing/guttering bits (soda straws, plastic coffee stirrers, streched sprue, and or insulated copper wire), bits of wall paper (google "printable miniature wallpaper")
I agree with the suggestion to add nails to your boards, but they look a little thick to my eye. put a 1/35th figure up against them, imagine standing on a ladder nailing that bad boy in place...
Something a but thinner would do it.
Looks great for a start, but as in life, it is the little details that really matter sometimes.
This is one of those times.
Mike