Once inside the outer walls, the inner walls will be a bit harder to take and the attackers face a small dilemma. The defenders will be stuck in the inner portion of the keep, the other world is sitting out there somewhere debating coming in and taking the keep behind you, and you have less space to move about. This presumes that the keep has an inner wall, some keeps do not, and therefore it just simply becomes a rush to the keep lord.
Ultimately, what will need to happen is another gate / wall needs to come down and the world needs enough forces to finish off the remaining defenders and take out the keep lord (who is very powerful) before the other world shows up and finishes off a tired and battered force.
The same works in reverse, letting another world beat the walls down and moving in after them is a viable and fair tactic. The lesson here is scouting is critical – knowing where the enemy is vital. Smaller groups need to be out in the world communicating on troop movements, denying resources, and tapping control points.
Siege weaponry is critical; Arrow Carts are great for taking out choke points and killing personnel, while Flame Rams and Siege Golems are needed to shred down gates. Catapults and Trebuchets are also needed to take out walls / gates from range. This applies throughout the entire siege. Let’s say you bust into the keep, keeping the enemy from reinforcing is critical, so placing an Arrow Cart at a choke point and having a small team man it is important.
Supply is crucial too; make sure there are people with Swiftness builds to sprint supply to all of the siege weapons. Without supply, a siege of a keep won’t be successful. Why? Well, a tower takes no time to capture, but a keep takes a while and the odds of it being unattended at astronomically low compared to a tower.
Resource camps are the easiest of the three to take. They have no walls, a few NPCs (five by default, 10 if upgraded), and can be taken easily with a group of five, unless upgraded. You’ll need to take out the Veteran Supervisor for the control point to spawn, once that NPC is down, and the camp is cleared, you can then move to take the camp over.
The biggest strategy is simply numbers. A group of five can easily take out a resource camp that is lightly defended (one or two players) and no upgrades. A second group of five is needed if the camp has been fully upgraded. Then, if it is heavily defended, it’s probably best to skip over that specific camp and move on to a different one.
Most camps are out in the open, so flanking the enemy (coming in behind them or to their side while they’ve engaged another group) is a viable strategy for knocking them out. Additionally, if you can’t take the camp, you can neutralize its affect by ganking the resource caravans that spawn.