What I think happened is right before Mars lost it atmosphere the oceans and seas of mars rushed over the land and ripped some of these floors into rubble. Lets take a look at one of the images that convinced me of this. This image clearly shows the floor of a crater ripped open like a tin can. Now rocks do not rip open like this only some type of metal.
If you are still following me then lets move on. After these metal floors were laid inside the craters it would have still been slick footing when it rained. Most of the activity occured near the top of the craters. Its here that you see in many locations and many NASA images the prefabricated square stone slabs. If you notice on the top of these squares you'll see a grove which would make it easy to overlap and secure to another square slab. I've seen various types and shapes but the square is the most numerous shape.
I've also seen small hooks and clamps coming out of the metal floor that could have helped secure these things.
As you get closer to the lip of the crater the walls tend to go straight up vertically. It was here that it seems they would hang faces of demons and other scary looking creatures. I believe the opposite was done here. From what I have seen it looks like many of these faces that appear to be carved out of stone may also have been prefabricated away from the crater and brought to the crater.
It appears like these big stone faces were built with hooks on the back of them and then hung in holes cut into the metal floor instead of hooks coming out of the floor. Here are a couple of images with those hooks.