The Plasma Turret is the most powerful defensive units in the game. It is fairly expensive, but well worth its price. It uses a powerful electromagnetic accelerator to fire a pulse of superheated, compressed gas, or plasma, at an extremely high rate of speed. There is no ship with Rapid-Fire against the Plasma Turret. Plasma Turrets are also useful for being able to destroy the widely-used Battleship in two hits, deterring attacks from fleets containing them. It takes 37 of them to destroy one Deathstar, but it would take proportionally more Gauss Cannons, because each time the Gauss fires it has to go through the Deathstar’s shields again, while Plasma fire is more concentrated in fewer turrets. Therefore, mass numbers of Plasma Turrets are really the only effective defense against Deathstars. It should never stand alone in a defense system, but should be backed up with other smaller defenses. The Plasma Turret is capable of bypassing the shields and the hull of any ship equal to or smaller than a Cruiser, destroying them with a single shot. This means large quantities of Plasma Turrets are capable of wiping away a large amount of Cruisers, discouraging the use of these ships to Rapid-Fire your missile launchers. They are better when accompanied by Gauss Cannons (to kill enemy fodder) and large amounts of defensive fodder like Rocket Launchers and Light Lasers (to absorb damage).
Large numbers of Plasma Turrets are also the only effective defense against a large-scale attack with Interplanetary Missiles (IPMs). While any determined attacker can “nuke” a defense away, the cost of building enough IPMs to destroy 3000 Plasma Turrets is generally not worth it unless the target has left a huge amount of resources on the planet.
Note: For pros and cons of defensive structures, see Defense Comparison