Four square corners, a roof-- it's a little old, but it is a building. Magnification is 400%, so it's a little pixelated. Lighting has been adjusted for clarity. Opportunity Sol 891.

Richard attached the following image(s):

From sol 894, we see another view. The side facing the camera suggests the possiblity of vines or other vegetation. Note how messy it is with many strands of whatever we are seeing there, with an exception: View the volute object (arrow). If the strands are pipes and hoses instead of vegetation then perhaps this is a small manufacturing facility. Also an odd bell-shaped object keeps popping up (box).

Richard attached the following image(s):