Upper Levels Building a two story cage, worried about them climbing out

My little Carly is very, well, adventurous and I want to build a second level since they like the dark. My cage plan is to have a 2x4-2x5 cage with an upper eating area and below the upper eating area is the sleeping quarters:shhh: Haha! Has anyone ever built a cage with the upper level just enclosed? I really want this built quickly and I know this is the best site for information because by a poll that Ly&Pigs posted, like 507 people have C&C cages(Ly&Pigs, if you did not want me to say this, please edit this and delete it) so if you have, please tell me or if you haven't but know, please also tell me. Carly and Sandy are waiting patiently and anxiously.