The basement is a 2x3 below the 2x6, where my single girl Nina lives. She'll be introduced to my pair Pumpkin and Rhoo very soon so they can all live together. When that happens, I'll put in a ramp to connect the basement to the 2x6 level. There are two thick dowels below the 2x6 for support. The front 3 grids of the basement serve as a door, being only zip tied on the top so they can swing open and closed. I keep it closed with mini bungee cords.