Early in the game, when you have en early expansion limit for an amount of time, you have many defensive strategies at your disposal, but some are more effective than others. I have found putting fighter/bombers in orbit around your base a very effective strategy for keeping out T1 and most of the T2 units. This is more cost effective and less time consuming than building T2 stationary defenses, though it is not a bad idea to make those too if you have enough resources later in the game.
Fighter/Bombers have the capability to destroy both land and air assaults, as long as they aren't huge. The Cybran Corsair is very good at this, having ASM's (Anti-Surface Missiles) that spread out, easily destroying large groups of pesky T1 units. It also has homing AA missiles, also wiping opposing air units from the skies. The Janus is good at this role too, but better at countering T2 units due to it's more concentrated bomb damage. The Aeons lack a Fighter/Bomber, but do have a fast interceptor, while coupled with the T2 Gunship may prove to be adequate defenses. The Seraphim have a Fighter/Bomber, but I am not sure how well it works compared to it's enemy counterparts.
In the beginning of the game, if there is an expansion limit, I recommend working on putting as many T2 mass extractors and T2 Power Generators up as possible, giving you easy access to resources later. Next I recommend work on your base, building air factories to bring your defenses up quickly, a land factory for engineers, shields for protecting your base from occasional slips through your defenses, and refueling stations to keep your defenses moving.
About five minutes before the game starts it is wise to commence building the Fighter/Bombers, for defeating initial T1 bomber attacks is essential. Five minutes and three factories gives a player about fifteen units before it gets exiting.
From personal experience this form of defense is very effective at keeping the enemy a good deal away from your base for a good portion of the game, at least until T3 attacks begin. That is the only downside to this, they don't last forever. That is why it is good to keep advancing your economy while building these units in an infinite loop, pre-set for patrol so they automatically defend your territory. You can and will get losses, but hopefully you have enough that any attacks will be destroyed before the end of the first pass, and you have enough to send a task force out to strike out at the enemy's outlying economy hot spots, oncoming forces in need of whittling down. Only in the final stages of the game when everyone has T3 units is it wise to stop the loop and use the resources for an experimental or some other game ending project.
1. Immediately work on bringing your economy to T2, power generators and mass extractors.
2. Build a base:
-3 Air Factories
-3 Air Staging Facilities
-1 Land Factory
3. Take the factories to T2 and continue to expand your economy.
4. About five minutes before the attacks begin start up your Air Factories, pressing the infinite build button along with the Fighter/Bomber. Also set the post-build factory setting so the aircraft Patrol in the areas attacks may come from, or maybe just all the way around your base.
5. Work on your economy more, along with a game ender strategy.
6. When it becomes obvious that the enemy is using T3 units and your economy either can take T3 units also or you need more resources, stop the air factories or give them a new job.
Remember that this is strictly a defensive strategy, use something else for an offensive one. If you send your Fighter/Bombers off to destroy the enemy base, you have destroyed your defenses and are open to attack until the factories remake the Fighter/Bombers, from personal experience I DO NOT RECOMMEND THIS!