At some point after Max had come to live with Grim and Gram after the death of his mother, his grandfather decided to modify the basement. He put some "cheap paneling" in the dimly lit room with only one window and the "bulkhead door," and soon Max had claimed the cellar room for himself and soon he was sleeping there. It was the perfect place for when he was brooding or angry or feeling sorry for himself and didn't want to be around Grim and Gram. It was Max's escape from the rest of the world and despite the "rug that smells like low tide" and the paneling that "buckles away from the cellar walls," Max does not complain.
I like it in the down under, got the place all to myself and no fear of Gram sticking her head in the door and saying Maxwell, dear, what are you doing? (Chapter 2)