You didn't say different, . . . so I assue that the walls are drywall. BAD !!!
First thing, . . . line the entire inside with 3/4 inch plywood. Put up the longer walls first, . . . then the ends, . . . then the floor, . . . then the ceiling. That makes it almost impossible to "kick" in the walls.
I would get a solid core door, . . . line the back side with 1/8 or 3/16 steel plate, . . . bolted at the edges, . . .
Beef up that front wall where the door frame sits, . . . use locking, sliding bolts to secure the door.
Beyond that, . . . it would take one DE-termined individual and some tools to get into that room.
Wish I had one like it, just to be honest.
May God bless,
If you can breathe, . . . thank God!
If you can read, . . . thank a teacher!
If you are reading this in English, . . . thank a Veteran!