defensive strategy early-mid game

I usually put a lot into defence early doors - some laser turrets, some pelters, air defences and often some lines of land barriers but am thinking now i put way too much into it. what better defence than a large mobile army of land units? i tend to rush lots of lobbers too and find that they never get used as my army deals with incursions, if they do come into play, it's generally too late for me.

what are your defensive strats?

As always "The best defense is a good offense" holds true in this game. Attacking your enemy first puts him on the defensive, wondering where and when you'll attack next, and causing him to build defenses instead of thinking about attacking back.

That isn't to say that defenses are worthless, but most games I've seen people stack defenses on one side of their base, or out of range of what they are trying to protect. Instead, you should spread turrets across your base, and they should be either just on the edge or slightly inside of your base. If you build them too far out, quick bots will run past or simply flank around the defenses. It's better to lose 1-2 structures but keep the turret alive than to have it die because it was the first thing targeted, so use your own buildings as a shield. Factories are generally hardier than other buildings, and the fact that they are building combat units generally means they need less protection. On the other hand, P-gens are top priority.

You shouldn't be trying to build so many defenses that your base is impenetrable. You only need enough to stop a small (10-20) group of tanks/bots from raiding your econ. Anything more is IMO a waste of resources that could be better spent elsewhere, and a competent opponent will simply attack elsewhere.

Also, don't underestimate your commander's capability to destroy units. He can easily take on 15-20 units on his own. Double clicking an enemy in close range causes him to fire his uber cannon (TA d-gun equivalent). With a few engineers repairing him he is quite formidable. Avoid going up against T2 units with him however, their damage output is substantially higher.

Put yourself in the enemies shoes, if he could see your whole base, where would he attack?