Batman would notice a small space behind the grand staircase at the center-front of the room. If he slips through the space, he would notice the wall and wallpaper over it. To the untrained eye, there is little that is suspicious with the sight, but to Batman, he might notice even in the dark that the colors on one part of the wall are off from the others. It's a secret door and behind it is a set of stone stairs that lead downward into the basement.