The Sentinel forces watch over planets and protect them from damage. They are robots that go around, and monitor wildlife. They are alerted by environmental damage, and attack if they see damage.

This is a Sentinel Walker. There is only one per planet. It is summoned during Wave 5 alerts. It walks on two legs, and is 30 feet tall. It can kick with massive force, shoot deadly laser beams, and has armor. It cannot speak and has limited sentience. On its destruction, Sentinel forces shut down.