The Critter Nation Add-On Unit (model 163) by MidWest Homes for Pets is compatible with the Critter Nation (model 161) and Critter Nation Double Unit (model 162). The Critter Nation Add-On Unit (model 163) will add more living space to your existing Critter Nation small animal cage and more room for your small animal to roam. The Critter Nation Add-On Unit has all the same great features of the Critter Nation cages. We included full width double doors that provide maximum accessibility for easy cleaning and feeding. The wide expansive shelf and full width plastic pan floor provide a generous play area and helps prevent dangerous falls. Designed with customer convenience in mind, we've included critter-proof dual-locking door latches that provide a secure, easy one-handed operation. Shelves include removable plastic trays for easy cleaning and are height adjustable so you can tailor the cage to your pet's needs. The Critter Nation Add-On Unit also has multiple attaching points for adding hammocks, tubes, toys and other accessories that are sold separately. Constructed from sturdy square tube frame and wire the Critter Nation Add-On Unit has an attractive gray-quartz hammer tone finish that is sure to blend with any decor. Best of all the Critter Nation Add-On Unit is easy to assemble and attaches to your existing Critter Nation Cage - no tools required. Please Note: Critter Nation Cage (model 161 and 162) is sold separately. You can accessorize your Critter Nation with the Ferret Nation Accessory Kits. MidWest Homes for Pets stands behind the quality of our products with a 1-year manufacturer's warranty.