(first guide! yay! watch this suck!)
For those who don't know, siege is, read the book. Oh, wait, nobody reading this played the original...
Siege is where there's two teams, one on offense and the other... well...
The round ends either when time runs out or the team on offense "captures" the base by touching the control switch. Then the players switch sides. The second team MUST beat the score set by the first team to win the match.
DEFENSE: the generators usually power the shields that guard the switch. People on offense will go for the generators just to get them down for a few seconds, so they will usually ignore you. So a shield pack usually isn't what you're going for unless the base is BIG and you're in the way. Stick with repair packs. When a generator goes down repair it as fast as possible.
That being said, you must know your base very well. Some bases have side or back entrances that allow people to capture the base very quickly. These are the places you need your juggernauts with shield packs.
As far as setup: Get out your remote inventories. These make it easier to deploy/replace/get repair packs. Get motion sensors and cameras all over the place. Deploy a turret in the back entrance so people can know when someone is coming. Most of the rest should go in the generator room or the just outside the switch room. Get one or two people in the switch room whose sole purpose is to kill anyone who comes near. Go easy on the mines to the gen room or anywhere people walk frequently. This will just piss off your teammates.
Loadout: Generally a good thing to do is set up with an Annoying Bastard loadout with an ELF instead of a blaster. You can use this to drain shields or ground lights, and it drains cloak packs, too. Therefore, this works whether you're in the generator room or the back hallway. Plus, in the back hallway, when you're carrying a repair pack, people don't have to go back to the inventories as often. As much as I hate to say it, Grenade Launchers are quite effective in defending those narrow hallways. SO ARE MORTARS.
Another good loadout is the standard Juggernaut Defense loadout. Good damage, you're indoors anyway, and your weapons have a nice spread. Plus you get the shield pack.
OFFENSE: Get those remote inventories out as fast as you can. This will help people who take damage in the aforementioned halls to go back a bit, repair, kit up, and get back to their ass-whooping.
Setup: Not much, you're on offense. The best you can usually do that's effective is get out your remote inventory stations. Some people deploy some turrets, but I've found that unless you're a god at staying alive in an enemy base, they won't help you much.
Loadout: Most work, actually. The plasma rifle is quite effective against generators. The only pack I disapprove of is the sensor jammer, you'll probably be in narrow halls most of the time. The defenders will think: Hm, who's the enemy, could it be the guy with the different color uniform and no red triangle above his head? hm...
Aside from that, shields are usually good if you want to bum-rush the enemy. Repair packs won't do you much good unless you manage to set up a base within their base. Cloaks can get you in, but usually not out.
Now for strategy. On defense, as I've already discussed, either defend that hallway or keep those generators running. Otherwise you might as well not bother playing. Get turrets up with motion sensors. even if the turret doesn't hit anything, the fact that it activates tells you where and when your enemy is coming.
On offense, there's often a generator that protects another generator. Send in someone to take out the first generator and have someone wait at the shield of the second generator. That one will take out the second generator, which usually powers the force field around the capture point. Also, the first people in should find any motion sensors and cameras and destroy them for any second wave that might be coming in.
This topic mostly serves as a chance to vent, I was annoyed at the ineffectiveness of the opposing team recently in a siege match. You people need to read this. It was painful.
For everyone else, I hope you learned something
Feel free to add anything you think I've missed. Maybe weapons that are surprisingly effective. Remember that most things in here are opinion.