Amazon is among the several companies working to make drone deliveries a reality. While they’ve already conducted successful trials in remote areas, making their first commercial drone delivery late last year, one of the many challenges still ahead for the retail giant is being able to make drone deliveries in dense urban areas in a safe and more efficient manner than regular road delivery.

According to a patent application filed in 2015 and published today, Amazon envisions building “multi-level fulfillment centers” at the heart of towns and cities to solve this problem. The patent proposes using these multi level centers to house drones, where they would be restocked by human workers, and then they’ll fly in and out of the location's many windows to deliver orders.

The patent describes three potential designs for a multi-level fulfillment center, which could support traditional truck deliveries and include a self-service area where customers can pick up items. It also includes proposals for reducing propeller noise and making drones more fail-safe.

In its filing, Amazon notes that traditional fulfillment centers are often located outside of cities due to their large size, such facilities are not feasible in urban environments due to the cost of land and limited square footage. Multi-level centers would be more fitting to bring fulfillment centers to sities.

Of course, patent applications are just that, and there’s no guarantee that Amazon will actually build these centers any time soon. It’s also worth noting this is not the first experimental patent Amazon has filed for its fleet of drone delivery vehicles. Last year, the company also filed proposals for "airborne fulfillment centres" that would be used as flying bases for drones.