Defense

Defenses are the structures built to protect the planet from enemy invasions. As they're attached to the surface of the planet, defense are not mobile nor removable by fleets, so you can't use them to raid.

You can find three main types of defense: turrets, missiles, and shield domes. Turrets provide cannons or projectiles to shoot the invaders' ships. They usually have a low armour and shielding. However, shield domes provide a great endurance to attacks, so they can absorb much more damage and give extra time to turrets and defensive fleets to shoot. Missiles can be used to attack players' defenses, but they can also be used to defend from oncoming missiles. The interplanetary missiles fired at your planet can be blocked with anti-ballistic missiles.

After combat, each defensive structure has a 70% chance of being rebuilt for free, unless it was destroyed by an Interplanetary Missile (in which case, it has no chance). Your defending fleet, however, will not be rebuilt. Destroyed ships from the defending fleet and the attacking fleet make a debris field. A Recycler can pick up the debris resources and add them to your resource pile.

The base time taken to construct a defense depends on the amount of metal and crystal the defense costs. Construction times are otherwise only reduced by Shipyards and Nanite Factories.

$$\mbox{Time(hours)}={\frac {\mbox{Metal}+\mbox{Crystal}} {2500 \times (1 + \mbox{Shipyard level}) \times {2}^{\mbox{Nanite Factory level}}}}$$.

Unlike buildings and research, it is not possible to cancel the production of a defense (this is also the case with ships). Once resources are put into building a defense, those resources can never be retrieved again.

However, also unlike buildings, defenses don't take up any fields. This means you can build as many defenses as you please, provided you have the resources necessary.

Strategies
For a list of defense strategies, see Defense Build Strategies.

Pros

 * A good defense will protect and allow you to build up your resources and fleet.
 * After combat, each individual destroyed defensive structure has a 70% chance of being automatically rebuilt for free.
 * Defense has NO effect on the number of developed fields on any planet.
 * Defenses generally are much more powerful than ships for the same amount of resources spent, not counting rapid fire.
 * Defenses allow you to spend less time at the computer, while still maintaining a competitive economical growth rate. In other words, turtles are able to pursue a more relaxed style of gameplay.
 * Defenses can't NORMALLY be turned into debris fields (DF) unless playing on a universe that has special rules "Defence into Debris" like "Hydra" which allows 30% Defense into Debris

Cons

 * Successive waves of attacks can easily erode defenses into insignificance.
 * There is a constant need to build enough defenses to keep other players' increasing fleets from breaking through your defense. In the case of universes where Alliance Combat System has been activated this is even harder.
 * Once built, defenses do not contribute to your economy in any way. They don't bring in resources, and they don't enable you to raid other planets for resources or crash other players fleets. For this reason, a turtle will grow in points at a slower rate than a fleeter, given that the fleeter uses the fleet and makes sure other players don't crash it.
 * When your defenses are destroyed during an attack, you lose the points and resources you put into building them (NB: there are a few Ogame.de universes, universe 10 on Ogame.ba and universes 39–40 on Ogame.pl where destroyed defense becomes part of the debris field in which case it would not be a complete loss of resources, but they could then be hunted for profit like ships).
 * Missiles targeted at specific parts of your defense can quickly leave you vulnerable to attack. To remain effective, large defense arrays requires the added expense of Missile Silos and anti-ballistic missiles to protect them.
 * A highly determined player or group of players can completely obliterate even a massive defense with large volleys of Interplanetary Missiles, resulting in huge point losses for the unfortunate target player.
 * A competent fleeter who sends his resources out while fleetsaving has little need for a defense. However, he should still build a small amount of defenses to protect his overnight production by making such a raid unprofitable once the attacker's losses are taken into consideration. If he does not, an enemy need only send a few large cargoes to carry away those resources every night (see farm).

Defence Calculator
See Tools

Defensive Fortifications
Rapid Fire

Note that Anti-Ballistic Missiles' (ABMs) Structural Integrity, Shield Strength, and Weapons Strength plays no role in the gameplay. (Interplanetary Missiles (IPMs) are used in missile strikes against defensive units.)

Note that the rapid fire in this table is what the ship has against the defense, not the other way around.

Efficiency of Defence

Note, these calculations DO NOT take into account rapid fire.