The Sentry Turrets in Portal are intelligent, stationary weapons built by Aperture Science. They are used within Aperture Science Laboratories to test Aperture Science Handheld Portal Device-wielding test subjects and, according to GLaDOS, military androids. Aperture Science Sentry Turrets function much like the sentry turrets used by the Combine in Half Life 2.
Appearance & Capabilities
The Aperture Science Sentry Turrets are plated with a shiny, smooth, white metal and have a single red "eye" in the middle of the body which emits a visible red laser, marking the turret's line of fire. Aperture Science turrets, like the Combine's turrets, stand on 3 legs. Aperture's turrets can also speak; they share a light, 3-year old-like, sing song voice with which they politely query their target's whereabouts, and ask it to come closer. They also call their target 'friend' on occasion. Upon being knocked over, they will tell their attacker there are no hard feelings, say that they do not blame their attacker, or simply utter a plaintive "Why?".
Aperture Science turrets have similar attack capabilities as the turrets employed by the Overwatch Forces. They have twin pulse cannons which have a very high firing rate. Aperture Science turrets are slightly more accurate than Combine turrets.
Tactics
Turrets, when idle, are in a closed position, the pulse cannons hidden and closed in the white plating which serves as the turret's armour. Upon detecting a target, the turret will open and immediately fire upon the target.
After being alerted, Aperture turrets make a variety of vocalisations, and normally ask for you to come out into the open, or wonder where you are. Needless to say, you should not listen to the turrets.
Portal's Sentry Turrets can only be destroyed by exposure to the Aperture Science Materials Emancipation Grid. One must instead use the Portal Gun to defeat the turrets, as they function like any other object found in the game. Once knocked over, the turrets will fire frantically before shutting down.