I played my first game of Planetstrike recently, and loved it. My Nurgle Force against a Tyranid force. These are a couple things I learned playing the defense.
If it's not in a bastion, it should be able to withstand a hit from the firestorm barrage. You can easily lose a large portion of your army if you aren't careful. I underestimated the effects of the barrage myself, and lost almost a third of my army (a predator with lascannons all around, a defiler, and a vindicator).
You should put ranged units into reserve if you can help it. Initial deep strikes into your base, or near the base need to be handled quickly. Having your units be able to handle that is key. But having units come in from reserve from a random location means that your close combat guys farther from the fight then you'd want.
2. Table Setup
There is no shame in making sure the table is setup however you'd like. Being detailed is important, but I think the most important part of the setup is leaving open space, and lots of it. Sure, the fighting will most likely take place inside your fort for the most part, but anything coming in from the board edge should have as little cover as possible. It's only natural. Commanders of forts and castles would clear away the trees from far around the walls ensuring a clear line of sight. Soldiers today scan the area where they setup defenses so they know all the nooks and crannies where the enemy might hide, and attempt to clear out as much space as possible. Even difficult terrain can be placed strategically. There is nothing wrong with clearing out a moat around your base. Obviously, all this could impede reinforcements coming in from the outside and hinder you as much, but you'll need to weigh your options yourself.
For myself, I still have a lot to think about in terms of setting up defensive terrain and fortress structures. Obliterators are excellent troops, especially if I can get them up high, so a double story bastion is appealing. In this case, this bastion is then worth two points, and becomes a desireable target to knock down. Of course, there are strategems that exist to hinder just that. Of course, even a normal bastion can easily accomplish this with a cleared out board. There is also the issues of placement of multiple objectives. If given the option, do you want to place objectives near one another or split apart. The split approach could be nice. It forces the attackers to commit or split their force. Against fast moving forces it's probably a weaker choice. I think in this case deciding your objective layout is something that should be decided based on the army type.
Against fast moving armies, lumping up the objectives close reduces the effectiveness of the armies speed. If they are near one objective, they are easily surrounded. They don't have far to manuever, and are generally weaker when pinned to a point. If the army tries to stay at range for a last turn attempt to rush the points, then all the better. Either way, keeping this army tethered to a small area reduces the effectiveness of their speed. Keeping this type of army out of cover is important, as being close and in cover will make your job more difficult.
Conversely, against a slow moving army, splitting up the objectives forces the opponent to keep moving the entire game. In this case, moving through difficult terrain can impede their progress. With these armies, range attacks are usually what they are looking for, so actually setting up some cover can help you just as much as them. Difficult terrain also slows down their progress.
These solutions seem sensible to me. That being said, I'd still have to try them out.
3. Table Setup, part 2
The other thing to consider when setting up your objectives is how you'll be able to keep your objectives should they be taken. You don't want to split your objectives up and then have no way to get them back. Of course, if you've split your defenses up, but not too far, you can use the strategem Escape Hatch to get troops out to the bunker quickly. After all, the Escape Hatch doesn't just have to be for escaping.
4. Consolidations and Holding Objectives
Be careful when consolidating troops near objectives. Try and keep your guys touching the objectives, and keep them spread out. Assaulting troops won't be able to get around your troops to touch the objective, but keeping your guys touching the objective can bring you a win if the attacker can't get into base contact. This worked for me exceptionally well in on long assault that found both the attackers and defenders barely making a dent round after round of combat. By keeping at least one guy in base contact with the bastion, I held the point.
5. Assaulting and Close Combat
Assaulting, and close combat in general, can be the key to victory for a defender. If you can keep the attacking units in close combat, they can't move. They must keep fighting, and that prevents them from capturing an objective. Even if they are touching an objective, a proper assault could force the attacker to move his models off the objective, at least denying him a point. This is great for the last turn in the game (well, you hope the last turn), where a properly angled attack can force a model off an objective. Of course, don't count on this happening often, but keep an eye out for it.
6. Tough Troops
You need units that can survive, and kill. This is why you've been handed 6 heavy slots, but this doesn't mean you need to use heavy units. Go for units that can survive hand to hand in combat, that are tough and long lasting. Plague Marines are my favorite for this, of course, I play Chaos Space Marines, so of course I would. If your troops are frail in close combat, you'd have to have a good reason to field them. Close combat will most likely happen for all your units, and you want them to survive (especially when you consider point 5 above). Remember, even if your unit slowly dies, keeping the enemy off the objective can give you the win.
This all means leaving home the fast moving but fragile units. Sounds obvious, sure, but it's very, very true and should not be forgotten.
Well, these are my initial thoughts. After a few games on the offense, I'll post my thoughts about that as well.