Here's some advice relating to pens (which were initially mentioned in Marine support).
Getting Marines inside a pen
Here are some methods of getting Marines inside a pen, once they're in the area. For anything more than a few Marines, you'll probably need a Warthog.
Letting Marines run inside
If you open a gap (ideally by moving a Ghost), Marines will tend to wander in and out; and you can usually encourage them to wander in by being inside yourself. You can seal the gap at any time, trapping some inside. Normally this method would only net you a few Marines, but with some pen designs they can be relatively slow to wander out once they're inside (which could depend on where the gap is), and in such a case you can get many Marines inside, though perhaps still not the whole squad. When resealing a gap by parking a Ghost, be careful not to do it when a Marine is standing close. A settling Ghost could easily kill him.
Backing over a low rock
Here's my usual method. If your pen includes a low enough rock, you can back the hog up so it's partly over the rock. When the gunner dismounts, he ends up inside. You then drive forward off the rock to pick up another gunner. Repeat until everyone is inside.
Importantly, if you have a particular rock in mind but it's not currently low enough, you can adjust it as follows. Fly back to trigger a tunnel checkpoint, but keep it delayed until just before the loading point you get on the way back, part way through the tunnel. As you cross the loading point, rock geometry gets randomized afresh, so maybe the rock is now suitable. If not, revert to try again (you'll be conveniently ready to cross the loading point again). Hopefully you'll get lucky within a few goes. Reminder: Marines can lose awareness of you when you go back to trigger a tunnel checkpoint, and this would make it difficult to pick them up in a hog; but that's fixed by making sure to get the attention of the enemy on your return.
When a gunner dismounts into the pen, you need to drive away promptly before anyone boards the hog. Hit the pedal just as the dismount animation is finishing; but not too early else the gunner gets some forward momentum and may end up outside.
You have a choice between using a normal hog or a Marine-retaining hog. I prefer a normal hog as you don't have to go around the back to get the gunner out. Just dismount, wait to hear his dismount animation, then get back in and pull away. Besides which, I've often got a sergeant on the chain-gun of the Marine-retaining hog, and I want to leave him in place. When using a normal hog I try to avoid having a seated passenger, in case I end up squashing him as I drive off. Actually, he'd typically dismount onto the rock, then get back in as you leave; but I still don't like risking it.
Backing through a gap
Here's another method, but it's more fiddly and carries some dangers. Pick up a passenger or two and gently back through part of the pen, temporarily displacing some building material. Do it in such a way that the gap remains filled so nobody can escape, and so that when your passengers dismount, they'll be inside the pen. Get out and let them dismount. Now gradually move the hog back out and the displaced material back into place, without letting anyone escape. If the displaced material was a single Ghost, you may be able to do all this just by boarding it and nudging the hog back out. Repeat until everyone is inside.
The main danger with this method is that you might squash Marines when backing in or when moving material back into position. Drive extremely slowly to minimize the risk. Another drawback is that during the insertion, a small gap may open up long enough for someone to escape.
Each time you want to get another Marine or two inside by backing over a low rock or through a gap, I'd recommend first drawing the remaining free Marines far away from the area so you'll have time to whizz back in the hog and do the insertion without having to worry about idiots running around nearby, trying to get themselves squashed. Dang fools! After all the work you've done, it would be annoying to lose a Marine like that. If you find that even that isn't giving you enough time for comfort, you could seal off the other Marines in the sunken area back near the underpass, so you'll have all the time you want. You can block them with a Ghost on the zigzag path.
Using a pen
When you've got Marines in a pen, defending from inside or just outside is an obvious plan. Another idea is to land well to the fore of the pen and progressively fall back as you weaken the enemy attack. Do a good enough job and maybe the Marines won't have much to do!
You can keep the Marines inside or release them - possibly with a Banshee blast as you approach. If you leave them cooped up and move away from the pen, you can potentially get a nice crossfire going; and if the enemy gets preoccupied by the Marines, you'll be able to wreak havoc without too much hindrance. Good fun!
When you fly into battle, you can potentially release penned Marines from distance with a well-placed Banshee blast, rather than having to get into a gap-filling vehicle to drive it aside. This leaves you more time to prepare yourself for the enemy. Valuable sniping time perhaps! It's my usual release method. Just make sure you pick your blasting spot carefully, else you may receive a less than friendly welcome from the Marines!
You can do this even when planning to land well away from the pen. Just divert briefly to send a Banshee blast over, then resume course for your landing spot. The Marines will join you shortly. Alternatively you could send a rocket over after you land. You can arrange some interesting situations by landing away from released Marines. The enemy attacking you can suddenly find themselves getting attacked from the side or rear by arriving Marines.
Incidentally, there's nothing to stop you setting up two pens if you've got enough material and Marines for the job. I haven't explored this yet but I think there are some fun possibilities. If the pens are close together it could be confusing for the enemy. Or perhaps you could initially defend at one pen, then fall back to another.
Here are some nice examples of pens to try.
Rock arc at back right
Around a tree at the back right (the 'right' being the side in shadow), there's a broken arc of three rocks which forms a convenient base for a pen. All you need to do is fill in the gaps. An obvious feature to exploit, this is the first place I used and is perhaps the easiest, especially when you bear in mind that no enemies spawn close enough to be any threat. On the minus side, this whole side of the battlefield is in shadow, making it harder to pick out enemies as they charge up. Still, you can always take the squad elsewhere once released. See Right side defence for a mention of this pen.
In regard to getting Marines inside, the leftmost rock can be low enough for backing a hog over. I always like to make sure that rock is low, so I can use that method and also shoot over the rock. I suggest you do the same. But if you want to use the 'backing through a gap' method instead, use the gap between the leftmost and middle rocks, having filled it with an upright Ghost. Back in at an angle so that when passengers dismount, they'll both be inside the pen. The hog should end up pointing towards the underpass or thereabouts. Get out and let the Marines dismount. Board the displaced Ghost and gently nudge the hog back out down the slight slope, so the Ghost is restored as the gap filler.
Rocks at the back
At the back there are two rocks on raised ground. A pen based around these makes a great defence position as the enemy come up out of the trough and along the platforms, and it's also an ideal starting place for taking Marines to defend elsewhere; left, right, or straight ahead into the trough. You can form a defensive arc using five Ghosts for example; three on one side of the rocks, two on the other. Later on, it's easy to release the Marines with a Banshee blast at either end as you fly in. Marines tend to group towards the middle, so it's fairly safe to blast the ends like that. You can see such a pen in BCM17 and BCM18.
A great bonus feature is the wide sill formed by the base of the cliff at the back of your pen. You can jump up onto it to get a majestic view of things, whether for sniping or rocketing distant enemies or wreaking havoc on ones closer up, who'll probably be distracted by the Marines. I like to put a spare rocket launcher up there on my throne. Cyborg life doesn't get much better than this!
The larger of the two rocks can be low enough to shoot over and to back a hog over to get Marines inside. That's my usual method of insertion here; I always take the time to make sure the rock is the one I want. It'll be wide and flat, and vaguely triangular when viewed from above. There's a small gap between the rocks, but I've never seen a Marine escape through it, so it's probably safe to leave unfilled.
Trees at back left
On the left side near the back there are two trees not far apart, one of which has a small rock beside it. String three Ghosts between the trees and you've got a good defensive front for a pen there, ideal for defending against an enemy charge up that well lit side of the field. That's makes for a superb battle (see Left side defence), so do make sure you try this location some time. I used a further five Ghosts and a Shade to complete a rather large pen in a roughly triangular design.
What about getting marines inside? At the back where my pen joined the cliff, I moved an object aside to open a gap, and stood inside. Just doing that, I managed to collect about ten Marines! They'd go in and out, but were slower to do the latter. It didn't look like I was going to be able to get any more however, so I closed the gap and tried to get others in another way, (except for Sergeant Johnson, left outside on a chain-gun). Unfortunately that turned out to be quite awkward. There's no convenient rock you can back over, and it doesn't help that the ground is sloping all over the place, which tends to make Ghosts slide down after the slightest nudge. I managed to get another few in but squashed one occupant in the process. Still, that gave me eleven inside, which wasn't bad for a first attempt.