Similar to the Corner Room Box, this version is slightly taller (so the walls above the platform are 8" tall) and deeper (7" x 7" platform).
I like the little step, too... keeps the piece contained plus gives you another surface (albeit a tiny one - only 5/8" tall) to add trim or faux brickwork to.
Cut from sturdy chipboard. This comes as a flat kit, but easy to put together with just a bit of glue.
I've assembled the piece with the corner on the left but it is possible to configure the pieces so that the corner is on the right side instead.
More info in the blog.