AI War:Gate Raids
How Do I Prevent Enemies From Sending Waves Against A Planet?
Q: Once I have control of several planets, it can be a challenge to keep enough ships on all of them to defend them all. How would I possibly hold and defend something like 40 planets?
A: When it comes to enemy waves, it's all about planet adjacency. Enemies can only warp a wave into a planet of yours if there is a warp gate on one of the neighboring enemy planets. So, in this situation what you want to do is destroy the warp gates on all enemy planets adjacent to the planet you want to protect. You can do this by completely taking over those planets, of course, but 1) if they are low value planets you might be increasing the AI Progress more than it needs to be for comparably little gain, and 2) if there is a much higher-level planet adjacent to your planet, you might not have the strength to take that planet until very late in the game.
The solution in those cases it to simply make a "Gate-Raid" with bombers and/or fighters (or something else that moves fast and hits hard), and destroy the warp gate while ignoring everything else. When the warp gate is down, get your surviving ships out of there! This can work with Mark I ships even if there is an Ion Cannon on the enemy planet, but you'll have to bring in more ships, and you'll take more losses. If you're playing on one of the higher difficulties, however, make sure you also check out this topic: Why Are Waves Coming In Without Warning?.
What Does Gate Raiding Really Accomplish?
Q: What does gate raiding really accomplish? I was reading that there are situations where enemy waves appear from nowhere - because while they have no immediate gate connections, they do still warp things to their planets and then walk them over.
A: The AI will always attack your planets directly if possible. That rule you are referring to only kicks in if there is literally no way for them to do that. So, this is a case where you can intentionally direct the AI to attack at a specific point or set of points by leaving them only a single or pair of ingress points. If you over-raid and clear out all the gates, then you're absolutely right, you've just defeated the purpose of it. But done correctly, it can be an immensely powerful tactic, and it's another thing I doubt I could win without on most maps. Granted, as soon as you take a new planet you have a whole new set of ingress points, and you shouldn't just automatically gate raid everything, but at least keeping the openings away from your core resource producing areas, and anything else you really don't want destroyed (advanced factories, for instance) is a really huge benefit. Just not carried to the full extreme.
How Does A Warp Gate Work?
In stepwise fashion, here's what a warp gate does:
1. The AI has built ships in some part of space "outside the galaxy." You don't know where that is, and you can never get there (maybe in an expansion).
2. The AI sends some of these ships as reinforcements to planets that it controls, but for the ones that it launches as offensive waves against you, you get the little countdown timer and the notice of what planet the wave is coming to.
3. The warp gate is what launches those offensive waves (but it is not required to sustain them -- so if a wave is incoming and you destroy the warp gate that enabled it, that won't help).
4. The wave, when it arrives, will most likely appear directly on the "far side" of a wormhole from the warp gate. It's like a slingshot effect, basically. In other words, if there is an AI planet with three wormholes that border player planets, it can slingshot the wave directly into any one of those three player planets through the wormholes leading from the AI planet.
4.a. As a corollary, if the players have taken over the AI's planet but left the warp gate for some reason, the incoming waves might appear right at the warp gate itself and attack that local planet (or they might still slingshot into adjacent planets, depending on what the AI wants to do). So that's why it's not good to generally leave warp gates sitting on your own planets.