Thanks guys. The ruins were an interesting challenge. I drew the base. Then I used some grunge brushes on a mask to really dirty it up and destroy areas so that it looked beaten up. Then I applied the layer mask and merged it down onto a white layer to have a greyscale image. I used threshold to get clean black and white image of the remaining walls. Then select colour gave me the walls as a selection and I converted it to a path.

Once I had that - the outline of all the buildings in their ruined state as a path - I used the photoshop brush panel to create a rubble brush. I took the standard hard round brush, increased the spacing, added some scatter and allowed for jitter on the size and roundness. This does a good job of drawing out a trail of rubble when you draw with it.

I used this brush on two layers. First with very little scatter to create the ruined walls. This more or less gives the white walls you see on the map. Then with much more scatter to give the dark rubble. A little more use of grunge brushes on a mask broke this up and give the underlying city ruins you see on the map! Oh, I used a straightforward drop shadow on the rubble walls to give a little depth. This was background texture so I wasn't going to go drawing all of those in by hand.

As for the size - this was a created as a print for full page at 300dpi, so the original is 2550 by 3300. However, the conversion from 300dpi for print to 100dpi for display on a monitor gives the same experience of the image. As this appears in a printed product (and in the pdf) I can't release the full 300dpi image I'm afraid. Sorry about that.