Imagine for a second that you are a terrorist who wants to blow up or hijack a plane. You know that once you get inside the airport, you will have to pass through metal detectors, bomb-sniffing dogs, and possibly a search of your clothes and luggage. How could you bypass all of those security measures? You could climb a fence or drive a truck to a sensitive area of the airport.
For this reason, the first line of defense in airport security is the most obvious: fences, barriers and walls. Tall fences that would be difficult to climb enclose the entire airport property. Security patrols regularly scan the perimeter in case someone tries to cut through the fence. Especially sensitive areas, like fuel depots or the terminals and baggage handling facilities are even more secure, with more fences and security checkpoints. All access gates are monitored by either a guard station or surveillance cameras.
Another risk is that someone could drive a truck or car containing a bomb up to the airport terminal entrance and just blow up the airport itself. Airports have taken several steps to prevent this. Large concrete barriers, designed to block vehicles up to the size of large moving trucks, can be deployed if a threat is detected. Loading zones, where people once parked their cars to get their baggage in or out of the trunk, are now kept clear of traffic. No one is allowed to park close to the terminal.