We put up a barrier so that the cats couldn't and/or wouldn't go down into the basement. In this picture, it was an old screen door we found. It is now a more stable and secure baby gate.

On the day we moved in, we put the cats in a room in the basement and baracaded them in. We wanted to be able to leave the outside doors open so that the movers could haul in our furniture without the worry of a cat or two escaping. Within minutes, Ogden broke out of the room in the basement and was free to wander the basement by himself. This is him trying for a second break-out into the main part of the house. He later broke back into the room we put them in. It contained the litter box and food.