World vs World in Guild Wars 2

For years, I'd been hoping for a good world-based PvP game. Mythic promised it with Warhammer Online's Realm vs Realm (RvR), but their open-world PvP devolved into objective-swapping, an inability to play due to poor computing resource management, and an inability to play due to stacking spammable AoE and creature control.  Now, NCSOFT hits back with Guild Wars 2 in what amounts to the most polished structured open-world PvP in an MMO.
World vs World vs World is a game mode accessed via the in-game menu (B), pitting players of three servers against one another in a massive battle for control of four zones. The servers change periodically, in what appears tantamount to some sort of server ladder. There are four zones: three Borderlands, the home territory for each realm; and the Eternal Battlegrounds.

 On each map, there are four icons that matter:
Supply Camps
These provide Supplies to players, as well as spawn an ox periodically to deliver Supplies to defensive emplacements (keeps/garrisons/fortresses). Supplies are used to make repairs to walls and doors, as well as to build siege implements. In other words, maintaining supply camps is essential for your war effort.  
Defending: Supply camps are a control point, so your objective is to keep opposing players out of the ring, and keep yourself in the ring. Use whatever CC you have available to you. Additionally, while idle, send a handful of players to escort the oxen through contested territory.
Attacking: Engage oxen as you encounter them on roads. They move very slowly (about as fast as a backward-moving player) and are easy to kill. As you approach the camp itself, you will have the opportunity to attack a veteran guard. Do so, and you'll get a friendly veteran guard that will stop enemy ox traffic. Once you push into the camp, you have to kill an estimated ten guards, at which point you all stand within the circle of control and take the point.
Keeps
Small holdings with a single entrance and one gate. These are extremely useful to hold, as most of them are within striking distance of both a supply camp and a garrison. Light defenses are usually sufficient against everything but a concerted effort, and there's little time to put up more.
Defending: The defender's job is simple: stall, and skirmish. One siege weapon, especially an arrow cart, should be more than enough to harangue attackers. Destroy weapons as they're built by attackers. From there, push players away, and continue to hold the point.
Attacking: You'll want to bring a ram to the party, at a bare minimum. From there, build siege weapons at a 1:1 ratio with the defenders. Start construction on one at a time, build it up, and move on. Cannon are preferable for their ability to target specific structures. Should you find that multiple weapons are destroying your weapons before you can get them built, start building multiples or move on, making them defend on another front and hopefully abandon the keep.
Garrisons
Medium holdings with two entrances and two sets of gates.  Very similar to a keep, but requiring you to manage your resources more carfully.

Defending: Two siege weapons at a single gate are more than enough to repel all but the most dedicated invaders.  If you're expecting attack on two fronts, see if you can't position one to cover both doors, reducing your need for weapons by one.  Put boiling oil on each door.  Instead of constructing on the outside during an attack, build on the second door, with the capability of attacking each entryway (again, depending on the garrison).
Attacking: You will need one ram, and at least one siege weapon per door.  Additionally, you'll want to match the defenders' siege weapons at a 1:1 ratio.  If they have people manning multiple engines, build multiple weapons at once.  Catapults are your ideal weapon here, because of their ability to destroy weapons and knock people off walls.
Fortresses
Large holdings, only found on the Eternal Battlegrounds, with three entrances.  Defend and attack in a similar way to a garrison, but expect a fierce battle.

Additionally, there are skill challenges and public quests, but they are self-explanatory, and I will leave you to explore them on your time. In each starting zone, there are additionally a number of merchants, including a repairer, crafting supply merchants, and a siege engineer, for all of your siege weapon needs. There are four types of weapons that make a difference here:
Cannon
A strong direct-damage single target weapon.  Wonderful for destroying siege weapons.  Not so good at attacking players.  The metagame may eventually change to have cannon blowing down the doors, if keeps tend to be well defended.
Defending:  Cannon are a tool to snipe siege weapons.  There's little use to put them on the defense, unless you've already got an AoE siege weapon.
Attacking:  Cannon can theoretically destroy doors and snipe weapons.  If the walls have few people, this is your weapon of choice.
Arrow Cart
A short range ground-target area of effect (GTAoE) weapon, the arrow cart is a wonderful anti-personnel weapon.
Defending: This is your bread and butter.  It's got enough range to drop the archers firing at your door, and enough AoE to clear them fast.  This is a weapon to bring when you don't expect the enemy to set up a siege weapon -- or you have enough skirmishers that you're confident you won't be dealing with one.
Attacking: Not nearly as useful on the attack, I would only bring one to a siege if the enemy had no siege weapons and the biggest challenge was keeping the enemy off of walls.
Catapault
A mid-range weapon between the cannon and the arrow cart, you've got a significant range that can reach quite far, still pack a wallop, and deliver a slight AoE.  It does require some guess-work, meaning your first few shots will likely miss.
Defending: This allows you to pick off siege weapons while still turning it on infantry.  Not nearly as precise as the Arrow Cart or the Cannon, but still extremely useful.  If you're not sure when or how the enemy is planning on setting up, this is your best bet.
Attacking: This also is a good way to clear out both defenders and weapons.  It's not direct damage, meaning door shots are primarily out of the picture.
Flame Ram
Your general all-around anti-door weapon, with an ability to spit an AoE around it.
Defending:  I've seen one of these set up on the inside of a door (albeit, it may have been a bug).  Would be a nasty thing to trap an attacker with.
Attacking: Essential.

This is the Borderland map.  Attackers come in from the southeast and southwest, while the defender spawns in the center above a garrison.

This is the Battleground.  Note that the center fortress has three possible doors, looking like it's a pain to defend without a coordinated battlegroup.