A shelter including a wall unit having walls. Each wall has a top edge, a bottom edge, and opposite side edges extending between the top and bottom edges. Each side edge is joined to a corresponding side edge of an adjacent wall to form a corner of the wall unit. The wall unit includes an access port in at least one wall. The shelter includes an upper rigid shell having a central portion surrounded by a channel for receiving the top edges of walls of the wall unit so the upper shell forms a roof of the shelter. The shelter includes a lower rigid shell having a central portion surrounded by a channel for receiving the bottom edges of walls of the wall unit so the lower shell forms a floor of the shelter.