Destroying the command station typically triggers a mass counterattack by any mobile forces on the AI planet. Mass assaults can also cause severe counterattacks. Engaging defending forces in a brawl can also trigger more limited responses.
This can actually be used to your advantage if your attack forces arent strong enough to roll over the planetary defenses, since a quick strike can lure them on to your territory to be crushed on your terms. The trick, of course, is living through it.
Without defences, you're basically engaging the enemies mobile forces. Most enemy ships are not serious dangers to forcefields except in vastly overwhelming numbers; Bombers and similar ships are a dramatic exception, and if enough make it to your command station you're in deep trouble forcefields or not. Your principle objective is to stop the bombers and bomber-analogues before they can reach attack range of your fields; other AI forces are worrisome in that they suppress your defences and kill your defending units. Every defence plan in AI War revolves around two aspects: killing them and slowing them down. Killing them is normally accomplished by your mobile fleet, supported by turrets - Turrets can almost never defeat serious attacks by themselves Slowing them down (so you have more time to kill them) can be accomplished many ways, but the most basic method is to use Tractor Beam Turrets on the portal the enemy is approaching through. This will halt the enemy in place entirely until they manage to kill the tractor turrets, which isn't easy. Even then the enemy group will be more spread out and easier to deal with.
If you're feeling truely paranoid, you can establish multiple layers of tractor turrets, use forcefields to make them resistant to most enemies, or even look into using advanced slowing methods like grav turrets, EMP mines, or engine damage.
Now, as for killing them...
Unlike most attacks, counterattacks consist of a varied mix of enemy units probably accompanied by Guardian ships. Since these ships have varied speeds, they tend to clump into discrete unit types on their way to your command station, which means that with a little elan and enough firepower you can cut them apart in detail. Your ships are typically faster than the enemies, so try to only engage their leading elements, and engage them with units that counter them for maximum efficiency. Typically this works as the fighters leading the enemy group, the bombers following, and the missile frigates bringing up the rear. Fighters counter Bombers, Bombers counter Frigates, and Frigates counter Fighters. So ideally your frigates are staying ahead of their fighters, pummeling them while staying out of range of their bombers; your fighters are mixing it up with their fighters and their bombers while staying out of range with their frigates; and your bombers are swooping around the side to pummel their frigates from the rear, outside the range of their fighters.
But, that's a lot of micromanagement. Just remember that the primary threat to your base is the bombers, and concentrate on having your fighters take out the bombers. Your frigates and bombers are basically there to support the fighters in this task, and are more expendable if need be to get the job done.
Particular turrets are good against different enemies, as well. Specifically, Missile Turrets are good against Bombers, MLRS Turrets are good against Fighters, and Basic Turrets are good against Frigates.
The wild card in this is the AI Guardians. I don't know what types show up in the tutorial, but theyre significantly more capable and usually more difficult to counter than the basic enemy ships. But, if you have a doubt, look at its Armor Type and look at your ships to see what's good against that armor type. Beware of annoying special abilities, like the Spider Guardian that hurts your engines and the Heavy Beam Guardian that has good bonuses against your turrets.
