"Once you build your first Ultrasonic Fence you will get a quest, \"SAFE BEHIND WALLS\", which will give you the choice of either extending the fence's range to 3 tiles, or preventing Trade Units from being attacked by Alien units. By two tiles of the city, it means of the center city tile. So aliens will still walk on tiles you claimed and are trying to work, they just can't get too close to the city itself."@en . "Once you build your first Ultrasonic Fence you will get a quest, \"SAFE BEHIND WALLS\", which will give you the choice of either extending the fence's range to 3 tiles, or preventing Trade Units from being attacked by Alien units. By two tiles of the city, it means of the center city tile. So aliens will still walk on tiles you claimed and are trying to work, they just can't get too close to the city itself."@en . "Ultrasonic Fence (CivBE)"@en . . .