It’s basically the lifeblood of the war. Without supply, mounting a vigorous defense or attacking a structure that is being defended is nigh impossible. You’ll need it to repair/upgrade keeps and towers and build siege engines.
Each player initially spawns with 10 supplies, which is also the maximum amount any player can hold. You can refill your Supply at Supply Depots located in Resource Camps, Towers, and Keeps, though I strongly recommend you refrain from taking the supply out of the depots of keeps or towers unless it’s really important, as they cannot generate supply.
One notable exception is when a keep is about to fall, as getting as much supply out of the depot is crucial because the supply trades hands when the keep is taken.
Supply is generated at Resource Camps and transported by Pack Dolyak Caravans, which spawn from Resource Camps and move on designated routes to allied Keeps and Towers, where they replenish the supply.
The amount each caravan carries depends on the upgrades of the Resource Camp. Defending these caravans and their routes can win or lose the entire battle, as they provide the only means for refilling the supply depots of keeps and towers.
You can use supply to repair walls and doors within Towers and Keeps. You can repair even if a structure is under attack, negating ongoing damage. However, you may not be able to repair multiple Golems/Rams so it might be better to save the supply.
Supplies have a second use in upgrading Resource Camps, Towers, and Keeps. The supply used for upgrades comes directly from the Supply Depot, and since players cannot refill supply depots you must have a good supply line in caravans and not be greedily subtracting from the supply depot constantly.
Finally, supply can be used to build offensive and defensive siege weaponry. All siege weapons require more than 10 supplies so you’ll either have to make some trips to and from a supply depot or have multiple players contributing.
You can build a siege weapon by utilizing a blueprint, which causes a build site to appear that allows anyone to use supply in order to contribute in the construction of the siege weapon. The various weapons and some of their supply costs are:
Offensive Siege Weapons
Arrow Cart (30 Supply): Anti-player weapon, fires arrows at the target on the ground, but does not damage structures.
Ballista (20 Supply): It fires a large bolt that does damage to structures and players, but is difficult to aim.
Catapult (50 Supply): Does ground targeted damage to walls, gates, and enemies, but only does moderate damage.
Flame Ram (40 Supply): Deals heavy damage to gates, but is very weak to oil and attacks.
Alpha Siege Golem (100 Supply): Essentially a portable ram, a player can load themselves into it and move it / assault structures.
Trebuchet (100 Supply): Long range siege weapon, great for taking the walls down on heavily fortified Towers / Keeps.
Defensive Siege Weapons
Boiling Pot of Oil: Does massive damage below the pot of oil and inflicts Burning. It must be focused before players can begin building rams / assaulting the door.
Cannon: It can fire three types of ammo (normal, Bleed condition, and Freeze condition) and works similarly to the Arrow cart.