Towers
Air traffic control towers can be over 100 feet tall and can be very artistic, like the the Sydney International Airport control tower in Australia or the Los Angeles International Airport Control tower. They are usually owned by the federal government, but some of the smaller airports are owned municipally. Most air traffic control towers look like a square tower that is usually 40-60 feet tall that is located inside an airport boundaries. Some can be many miles away from an airport and these are air traffic control centers which are used for ground to air comunication while the aircraft is in flight. They keep planes updated about their current position, in relation to other aircraft in the area. Both items are accepted in this category.
Here are some examples of some of my waymarks that have ATCT's inside the airport boundaries. A full control tower has a 360 degree view of the airport. Although a full ATCT's would be nice, it does not need to be a full one as all are accepted.
Many towers are very artistic. The Sydney International Airport is a very good example. This is one way you can waymark while you are on vacation. Other categories include "Inside Airports" and "Official Local Tourism Attractions" are great examples of the other categories you can achieve while on vacation, even this one!
Some of the larger airports have more than one control tower so each separate one will be included as an individual waymark into this category.
Centers
Now, some of the ATC centers are located outside of an airport. For example, the Los Angeles Center for air traffic control is located near Edwards Air Force Base in California. Even though there is no tower there, just a business building, it counts for being waymarked into this category because whether on an airfield or off, they both have the same basic function of getting aircraft to their destination with little problems as possible.
Air traffic control centers, although less common than the towers everyone sees in airports, are ran 24 hours a day, seven days a week. Unlike some air traffic control tower which closes when the airport closes, centers are kept open for longer periods and never close. They are much larger than the towers as they cover a larger area than just an airport. Their area of control is usually about the size of small U.S. states put together. It is quite impressive how large the facility has jurisdiction.