Yes, that is a litter box in the corner and yes they use it! They don't use it like a cat would. They really just use it because it is in the corner they want to use as a bathroom. The trick is to put something they would not want to defecate on in the other corners. This "trains" them to go in the litter box. I place the houses, food, and water bottle in the other cornes. I use basic non-clumping cat litter for the litter box. Guinea pigs do not jump, so they do not need a lid on the cage. They do climb or hop on top of their houses. The cage is still high enough that they cannot get out.
I use Carefresh Natural Pet Bedding to fill the cage. Before I place the bedding in the cage, I line the cage with chart paper. This makes it easier to clean at the end of the week. I also use a dust pan to get out the old bedding.
I hope you liked this peak at my piggies' palace! You must check out some of the creations people have made! This lady has a guinea pig room...those pigs are living the life!