If you get an Ultrasonic Fence but have Aliens near your city, your Citizens will be prevented from working tiles near them. If an Ultrasonic Fence prevents Aliens from getting within 2 tiles of a city, why are these tiles "Blockaded" by Aliens 3-4 tiles away from the city? The people "working" these tiles are under absolutely no threat, it doesn't make sense that they get scared and can't work these tiles. I mention this because in my last game I founded a city with a ton of Algae around it. There was a couple Kraken trolling around nearby, and it was much too early in the game to effectively deal with them, so I got an Ultrasonic Fence instead. Too bad the city is still stagnate and can't grow because I can't work the Algae within 2 tiles, just because that Kraken looks scary or something?