We took a night train from Cairo to Luxor...sleeping sitting up (again) for 4 of the 6 nights of the trip (not the best idea...neither is going to the Sahara Desert in the middle of the summer, but we all learn our lessons sometime.)
The Valley of the Kings was built after the pyramids, but they still used the 'natural' pyramid formation of the mountain.
They buried their kings and treasures here, camouflaged inside the mountain, so that thieves wouldn't detect the tombs as easily as they did the pyramids.
There are 64 tombs that have been discovered here, the most famous is that of King Tut in 1922. It was found untouched and with all of its treasures still inside. The goods have since been removed and placed in the Egyptian Museum in Cairo. It would have been wonderful looking at all of the stuff, but the museum had no AC and it felt like it was 500 degrees inside. There were around 74 kings during this time, so there still might be a few tombs that haven't been discovered!