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 Cage and offer more room for your small animal to roam. The Critter Nation Add-On Unit has all the same great features of the Critter Nation Cage. 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 help 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. Includes appropriately angled plastic ramp that is specifically designed to ensure pet safety and protect your pet's tiny feet. Constructed from sturdy square tube frame and wire the Critter Nation Add-On Unit has an attractive gray quartz hammertone 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 also accessorize your Critter Nation with the Critter Nation Accessory Kits. If you have any questions about our Critter Nation, our customer service department would be glad to help you. MidWest customer service can be contacted at: 1-800-428-8560, available 9:00 AM - 4:00 PM, Monday - Friday.