This is a comparison of the pros and cons of each defense.
The Rocket Launcher is a good, cheap, early game defense. In later stages they’re only worthwhile as fodder, and massive numbers can absorb a staggering amount of enemy fire in a single combat round, allowing heavier defenses more time to do damage to attacking ships. When under attack, excess metal is often dumped into Rocket Launcher production to prevent it from being plundered.
Pros: Cheap, does a better job as fodder than Light Fighters at half the cost.
Cons: Unbalanced cost, supremely weak, Cruiser’s Rapid-Fire (RF) cuts these down like grass.
The Light Laser is a better defense than Rocket Launcher. These are used more than Rocket Launchers because they are more balanced in cost.
Pros: Better firepower and shields than Rocket Launcher, balanced cost.
Cons: Light Lasers are much more susceptible to Destroyer attacks than Cruisers, as Destroyers have Rapid-Fire against Light Lasers.
The Heavy Laser is “heavier” than the Light Laser, but as for pure firepower, you’re better off buying 4 Light ones. However, advanced players may use them as a pure armor fodder for IPM attacks. With IPMs only counting armor vs weapon power, you can absorb many more IPMs with fewer losses with sufficient armor tech and heavy laser counts. In addition, Destroyers have Rapid-Fire against Light Lasers, thus making Heavy Lasers a worthwhile defense against more developed players.
Pros: One Heavy Laser can take out a Light Fighter in one shot and “can” damage Deathstars if the defender’s Weapons Technology level is 10 or more above the attacker.
Cons: For the firepower they offer, Heavy Lasers are too expensive.
The Ion cannon exists mainly as fodder due to its high shield strength but is lacking in actual firepower. They are often used for getting rid of spare crystal, especially during an attack.
Pros: High shield power making them very good fodder, and are great against all ships below the Battleship especially Cruisers and Heavy Fighters.
Cons: Not Very cost-effective, and costs 3x as much crystal as it does metal.
The Gauss Cannon is the second-to-best defense out there. With high shield and attack power and generally low price, they can be constructed in large numbers.
Pros: Can destroy a Cruiser in one hit and it’s attack is equal to that of a Battleship. Gauss Cannons have a very balanced price for the firepower they offer, thus making them highly efficient.
Cons: Like most large defenses, it can be ravaged by large swarms of fighters.
The Plasma Turret is the most powerful defense mechanism in the game. Its firepower and structural integrity is very high, and when encountered in high numbers, Plasma Turrets can be a tough nut to crack.
Pros: Destroys Battleships in one hit, and 7 Plasma Turrets can draw with one Deathstar. Massive numbers make IPM attacks unprofitable.
Cons: Expensive and unbalanced cost. Deuterium cost inhibits building large numbers of these.
Hint: Keep about 1-2 Anti-ballistic Missiles per turret (topping out at 50-70 missiles) as Plasma Turrets are the main reason players attack with Interplanetary Missiles - until they exist in massive numbers.
A common strategy for defense building is to burn all unused resources on a planet before going to bed or leaving the game for an extended period (resources that can not be “saved” by loading them onto a Fleet-save Mission, often referred to as “Resource-saving”). Add defenses to the build queue, starting with the maximum number of the most expensive defense possible, then the maximum of the next cheapest defense possible, and so on, ending with the maximum number of Rocket Launchers. This removes any excess resources that might invite attack overnight. Following this strategy every night will rapidly build up a good defense.