I have been considering doing an outside pen for my cats so they can enjoy the outside at times. One that they could go to themselves through a door or window would be best I think. I have heard that some people have had extra critters come through the pens and into their house! That is what I don't want! We live I in New England and there are a lot of critters that are the same size as cats and they might like to check out the inside of our house. Have any of you ever tried this? Does it work and how if you did do this did you build it? My cats are strictly indoor cats and love the windows so I often wonder if they would like the fresh air and the outdoor environment if I provide a safe way for them to do it.