Aside from the appropriate locking mechanism from the inside that you would need to add, you might consider searching Texas Tech's website for their information on storm rooms/safe rooms/tornado rooms. They have been doing a lot of research on what it would take to retrofit an interior room to withstand tornadoes and their debris. As I recall from seeing on TV, their tests involve shooting a 6' long 2x4 at walls at something like 250 miles per hour. Cinder blocks and regular brick walls did not do well.
If you live in one of the plains states or in an area where tornadoes occur, you might consider outfitting a room that would do double duty as an intruder safe room and as a storm safe room. Many of the people who put these in do so with a closet, so you could have triple use. In the cases where houses have been hit and they did have a safe room, often the only things they have left are what were stored in the safe room.