By yahtzee414 and THE DOMINATOR CLAN

Big B's Citadel is a Standoff based variant with high amounts of fun with low amounts of trouble.

Choose Your Side
At the beginning of the game you must be in two teams. What you do not know is what your team will be doing. You will either be defending a large air-based fortress with large weapons stockpiles with pure force. Or, you may be attacking the fortress on the ground, constantly coming out of hiding and blasting away at the castle's owners with various heavy weapons and Sniper Rifles scattered along the battlefield. Or, Mount up on a Warthog and drive into the Man Cannon to shoot yourself into the enemy base and take it by force.

Attackers
Usually in Skycastle style variants, you always want to be on the castle, accessing the large amounts of weapons which your counterparts on the ground do not have. But on Big B's Citadel, you are given much more opportunity to have a good offensive when you are on the ground...

-You can hide out in the base below and pick up one of the many Spartan Lasers or Missile Pods in the mountains of crates. Pick off your enemies one by one.

-You can stay on the battlements on top of the base and use the Sniper Rifles and Plasma Cannons to bring the heat to your enemy.

-Jump on a Warthog, Mongoose, or Ghost and board your enemy through the Man Cannon.

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Defenders
Take the position that thousands of Skycastle nuts before you have coveted; on the base. Access the large weapon stockpiles or stay in the hangar and blast away approaching enemies with the Gauss Warthog.

On the first Deck you must defend the hangar, the landing spot for Man Cannon commandeering enemies. If the going gets tough, access the teleporter that leads to the top of the citadel. Also, you can grab a Sniper Rifle and ride up the Grav Lift elevator, leading to the second deck.

On the Second Deck you have two major options. You can either go left and go to the Banshee and blast away your enemies from the air, or veer right and enter the arsenal tunnel, grab a weapon and step onto the first Sniper Deck.

Take the teleporter on the first level and and you will end up on the top level. Here you can take a break behind the safety of the shield door and recharge your shields or strategize. Or you can take the teleporter the the other edge of the map to the second Sniper Deck.

UPDATE: There has been an issue of spawn killing by the defending team using Sniper Rifles and Gauss Warthogs, this has been fixed.
UPDATE 2: There will be a sequel of sorts of Big B's Citadel which will debut in a matter of days. It will be called "Battalla Point".

Click Here to Download Big B's Citadel to Halo 3
Bungie.net : Bungie Online : Halo 3 File Details