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.