They appear as glowing red symbols in the ground, to keep someone, or rather something at bay. It is unclear who puts them down, as Vireka's adverse reaction to the stench of the Beast Pens implies he isn't down there much to do so himself.
In The Forges of DawnEdit
When Vireka gives Uhuru a tour of the Black Mountains, he takes her to the Beast Pens, where the Beast Tamers keep the Hounds. Hoping to shake her of any traitorous notions she may have, they tie her up and leave her in front of a houndified Tarute. Jesiza warns her not to move around, lest she smudge the Wards and let a hound loose to kill her.