To everyone who says its empty,
I asked my brother if he wanted to make it more enclosed and he said he might eventually. However you shouldn't be critisizing his and mines build like that because we used a lego manual which we obviously didn't alter. Planning for our own add-on's doesn't come in a manual for us... it takes a lot of time and effort okay? (maybe not for those of you who have 6+ hours a day to devote to it but we don't)