A horse becomes cast when they lie down next to a wall with their legs facing away from that wall then they roll over onto their backs and continue that roll until their legs now face that wall (they roll over). Instead of having room to position their legs to get up or to even roll back, the wall now becomes a blockade (see the video).  The horse is unable to rise or to roll back.  They are stuck.  Horsemen call this casting.  Casting rails attached to the walls, like you see in the gallery, give the horse the ability to secure their hooves to the wall and push their body away.  They can now get up.  They are an inexpensive addition to any stall.