Amazon has filed a patent for a “multi-level fulfillment center” for delivery drones. The building would be constructed upward instead of sideways like a traditional warehouse – an arrangement enabled by the fact that flying drones could be deployed from any level of the structure. Amazon’s patent application with the United States Patent and Trademark Office says that the multi-level drone barn could be situated “possibly in an urban setting, such as in a densely populated area.”

The patent, found via GeekWire, details how unevenly spaced, automated doors would make sure the drones are secure in and around the building and prevent the entry of unauthorized vehicles.

Amazon’s drone program, Prime Air, demonstrated autonomous delivery in public in the United States for the first time at an Amazon-run conference in March.