Atop the kofun

 This picture was taken from the triangular area and looking toward the round part. The door is just out of sight in the shadowed area on the right.

 This was taken from the top of the round mound looking back at the spot where I was standing for the previous picture. Here you see that there is a flat shelf running all the way around the kofun but a meter or so below the top.

This is the round mound from the side opposite the door. The horizontal line is the flat are I mentioned above. Notice it does not actually go all the way around. It does not go around the top of the keyhole shape.