As something of a turtle type I thought I'd write a little about defences and the various merits of the different types.
Lots of players will tell you that you don't need defences, and they're sort of
right ... sometimes. If you can be sure you're always going to be able to fleetsave and mineral save then strictly speaking you don't need defences. However sooner or later you're going to mess up, real life will insist that you miss your fleetsave, when that happens it's comforting to know that you might just have enough defences that the attacker passes you by. Remember this game isn't about being so tough they *can't* attack you, it's about being just a little less attractive than the next guy.
Missile Battery: Obviously the cheapest and weakest. Avoid building too many of these early on (when Poseidon can rapid fire through them), laser are better early but you can use them to bulk up your defence shell using them as chaff. Probably wise to never have more than 50% of your defences being missile battery. It's very easy to use up your left over Ore on these at the end of the day.
Laser Battery: Better than missile battery when you're small but vulnerable to Prometheus rapid fire when you're bigger. Useful to bulk up your defences as chaff. Again advisable not to have more than 50% of you’re defences being missile battery.
Pulse Cannon: Has a good chance of destroying an Atremis in a single shot. This makes the Pulse cannon very effective at destroying attacking chaff and making an attack unprofitable (even if it wins). The best hull for cost ratio makes them least vulnerable to IPBMs (InterPlanetary Ballistic Missiles) but always build ABMs (Anti Ballistic Missiles) to protect your defences. Good defence for early players, vulnerability to rapid fire from Ares means these should be used in moderation by stronger players.
Particle Cannon: The large shield gives this defence some survivability especially against chaff attacks where multiple hits *per round* are necessary to take it out. Alas the high crystal cost makes them painful to churn out in large quantities. Later in the game their vulnerability to rapid fire from Ares means stronger players should use these in moderation.
Gauss Cannon: Great for mid level players, it's immune to rapid fire (except from Zeus) which makes it good for breaking rapid-fire attack runs. Often overlooked by large players who favour one Plasma to three Gauss there is still a place for Gauss in a well-balanced defence.
Plasma Cannon: The big daddy of defences. Capable of destroying battle ships in a single shot (sometimes) the Plasma is a great deterrent but having to rapid fire itself it's vulnerable to being overwhelmed by chaff attacks. Ensure you have plenty of chaff yourself to protect the plasmas. Pulse are a good compliment to the Plasma.
Decoy: Wonderful, as soon as you can build one on every world. It's cheap enough that it can be easily replaced if destroyed and it forces an attacker to send a few big ships with the freighters (when placed on a mining world), the big shield just soaks up the damage even from multiple hits. There's a good reason why you're restricted to just one, it's too good to let you have more than one!
Large Decoy: Similar to the decoy except it's large cost means most people can't replace it easily if destroyed. For homeworld defence it's somewhat lost by the fact that you should have so many other defences that it's % change of getting hit goes down. It's actually less useful than the normal decoy but should still be built on Homeworld/fleetworlds.
As with so many things in this game the answer is balance. There is no single defence that does everything for you, your attacker will adapt to whatever defence you have so you need to have some of ever
