Since moving to a new home close to a Bald Eagle nest and a busy road, I've kept my cats indoors.  After two years they've adjusted, but the male cat still tries to get outside.  After easily assembling this, we call it "Aggie's Porch", I placed it on our deck next to the sliding glass door.  I can crack the door and Aggie loves the added space and openness.  It also gets him more into the sunshine.  When we use the deck, we can close him inside and wheel him over to us.  It's tall enough so that we can use the top door at a window also.  It was an expensive investment for a cat, but all his purrs make it worth it.  

Pros: Easy assembly.  Options for placing the shelves at different levels.