Day 1:  Pink stuff, cut to size and glued in layers...
After laying out a hardboard base, I started placing blocks for the building.
The pyramid used two 15" squares with a 5" gap as a base, then 12" squares and finally a 9"x14" layer on top.
I used 2" thick polystyrene because of space concerns for the eventual location, but I think each layer should actually have been 2.5 to 3 inches thick.

The skull was carved out of some 2" polystyrene with a hot knife and wire. As a note: Eyes should be centered in the skull (height). The distance between the chin and the eyes and between the eyes and the top of the skull is the same. The "brace" for the skull and the stairs were cut to size and then a layer of spackle was spread over the entire piece and used to fill in gaps left between parts.

While that was drying, 2 small (6" - 3") and one larger (9" - 6") platforms were cut out of 1" polystyrene, glued and random blocks were cut "out" to show time, weathering and random damage. These were also covered in spackle.

A small piece of hardboard was cut into a less "squared" shape and an extra piece of pink stuff was sliced into a nice looking hill. The goal for this was to have a sand hill curving around a spot which may eventually hold a small oasis-pond. The trees were made by using 4 strands of 14 gauge wire, twisted and wrapped in 1/2" masking tape. After spraying brown, the pieces were glued to the oasis base and the base and roots were covered in a thick layer of scenic cement and sand. The trees were then dry-brushed with a "buff" colour and fake fern leaves were hot glued to the branches.

The last two pieces were made from plastic cake pillars. (The hot wire actually cuts through the plastic pillars fairly easily.) I used a few 3" pillars and some 5" ones for the longer "straight" pieces rather than the flared base and crown. The pillars were hollow, so some spackle was added in order to fill them out.
Finally, the (now) ruined pillars were glued to the base and sand was cemented around them.

Day 2:

Well, everything pretty much dried over night and a coat of brown paint was sprayed over all the sand and spackle, then a coat of "squash yellow". When that was dried, I used one of those "sandstone effect" sprays, and finally a coat of beige/buff. Although the pictures don't really do the result justice, there is some depth to the colours and the deeper cracks show yellowed and brown depths. Because of the angle I sprayed at, the pillars actually have slight "shadows" as well.

Last bit...

I used some strips of plasti-card which were glued on the edges of the building and then painted a blue-grey. This helped to define the edges of the building and also emphasized the parts that were crumbling.
The "headband" over the skull was also done with plasti-card and the gap filled in with yet more spackle,

Total work time: 4.5 to 5 hours
(with additional time spent waiting for things to dry.)