sigma isnt really "attached to the walls"
he comes in from the sides.. and goes back out
it looks almost like hes floating in space really.. he has a lot of organic wiry frame thingies holding him up inside

ill make you a picture so you can see what i mean by the obstacles and spikes and whatnot.. as well as the way he looks when he comes inside.. when i get time ill replay x4 and give you some pics of final sigma to refresh your memory
but he has a lot of "parts" at all times coming at you blocking your way or hurting you