It really is bigger on the inside. Seems to be more of a floating pleasure barge than stalwart defender against alien threats.
The Control Room Through the Years
The most consistently seen part of the TARDIS, this room is used by the Doctor to single-handedly steer a time and space machine designed to be driven by no fewer than six simultaneous pilots.
Source: ngfagossamer.tumblr.com
The Library
Visited by Clara in "Journey to the Center of the TARDIS," the library was often mentioned, but this was the first time it had been seen.
Source: bbc.co.uk
Another Angle of the (Impressive) Library
Source: raggedymans.tumblr.com
The Art Gallery
Shown off in the episode "The Invasion of Time," the art gallery cleverly doubled as a hidden ancillary power station. A switch on the Venus de Milo shut off the illusion and allowed access to the emergency power.