Quite a number of them have been killed in a war, which can be seen be the various wounds they have suffered. Then again, most of those are of the more bottom layer of bones, many of these are dwarven by nature. The skeletons atop of them vary in type and size, quite a few look to be humans, deep orcs, elves or drow, and the occasional ogre. All apparently slain by weapons or minor (i.e. non-blasting) magic.