I have a room which is 16x16 x 2high, the floor is made of dirt, it is only accessible by a ladder which you need to jump to from a few blocks away to hit and climb up. So no mobs can access the ladder. My room is lit with torches so all areas of the room, in debug screen to verify, are at light level of 10 or higher. So far zombies, mobs and skeletons have spawned in this area. There are no spawners near by.