There are so many times the keep I'm in is hit with an attack where the attacking players really know what they are doing, and I see my fellow defenders making serious mistakes in attempting to defend. So this is my attempt to help new PVP players defend their keeps. The good news is the way sieges are designed means if you do this right, you as a new player can be a HUGE help in defending your castle. Side note, I'm only a brigadier rank, so this is truly a beginner's guide. I can't speak to elite meta siege guilds and their best starts when 20+ people coordinate well. Second, while somebody who understands this guide will be benefitted in other areas, I'm really describing defending the outer wall.
Your priority is to keep them outside. The way you do that is protect the wall or door. Player spells and attacks can't hurt the wall, siege can. Your first priority is to dismantle their siege, not try to gank a player.
1. TOP PRIORITY. Stop the battering ram. Nothing will get the enemy into the keep faster than a battering ram assault on the door. If your keep is under attack, your first response is to see if they are going after the door with a ram. If they are, you have precious few moments to get a counter in place, if its not already too late. Good news, you have the advantage on this if you act extremely fast. You pour down crazy amounts of oil and melt them. Simple as that. On an outer wall door defense, there are four good places to put oil. On the very top of the gate tower, there are two little unwalled notches you can step out on, those are meant to place oil pots and rain the oil down as fast as you can. One man can place two oils and run them both. You need a minimum of two oils for a good ram defense. One will not stop them. Three or four oils is much better. If somebody mans one of the oils you place, don't be offended, rather your new goal is to dump your oil at the exact moment he dumps his. This is so you can get better burst dps on the people below. One floor down on the gate tower, is a grate looking down on where the ram will be. You can set up another two oils here, IF you spread them out, don't place one in the middle. If you place one in the middle of the grate, everywhere will say "too close to other siege weapons" and you've just robbed your team of an oil slot. The enemy can't handle four oil pots continually raining on them. If some can...I promise they are the exception that proves the rule. Do this first, make sure somebody is pouring oil onto the ram. Do just this, and you've bought time for everything else I'm going to say. As a side note, the current meta attack strategy has as many as five or six ballistae all crammed around the ram. So oil gets all that crap cleared out. EXTREMELY IMPORTANT: Do not place ballistae in the oil spots! There are only four good spots for oil on an outer door defense, and this is TOP priority. Ballistae can be placed tons of places. I want to cry when I am placing one oil and an ally drops a ballistae on the second upper oil spot. You are handing them the keep when you do this.
2. Second, destroy their Trebuches and Ballistae. If somebody already has the oil covered, your next thought should be this. If there is a ram and no oil, YOU make sure the oil is covered. If somebody else has it, then you want to take out their other siege weapons. A good attack force will have each of its attackers either manning two or three siege weapons each in an outer ring of players. If you stopped their battering ram they then turn to doing this exclusively. This is slower, especially attacking a wall but can still rip down a door quickly. Good news! you again have the advantage. Each of their siege can do from 1-4k damage per strike against the door depending on what they are firing. If you use a stone ballistae...you should be using a stone ballistae...your attacks do 12k damage against their siege weapons. So if you are attacking with a stone ballistae against their siege weapon, you will tear them down way way faster than they damage your door. BUT !! It gets better. If you will NOT aim at their siege weapon, but rather aim at the ground in between two closely placed siege weapons, you will damage both. Your ballistae is an AOE attack, use it that way! I often can be hitting three siege weapons per shot with my stone ballistae. And there will almost always be at least two close together because somebody is going to be running two of them and they don't want to run super far to go back and forth. Four stone ballistae being fired for max effect like this, can fight off a far superior number of siege weapons. So think like this...first, where can I shoot to where I am hitting the highest number of siege weapons with splash damage with my stone ballistae? Second, if you've beaten them down to where you are only able to hit one at a time (a sign you are successfully defending) then you want to hit the same siege weapon that other people are targeting so you collectively tear it down as quickly as possible. Also, if you are down to attacking one at a time and one of them is firing glowing blue attacks at the keep, choose that one because its doing more damage. But the biggest mistake I see is people directly targeting enemy siege weapons, thus only damaging one, when they could be hitting two, three, or even four at the same time by aiming at the ground between a cluster of them. Also, the BEST way to take out a revive tent is with your ballistae if they've mistakenly placed it too close to your wall. Seldom happens, but don't miss out if they do.
3. On ending the siege in favor of the defense. Understand that your goal is to stop them from tearing down your wall. If they are making progress on that, you are seriously messing up if you are just dancing around casting spells down at them. And you are seriously messing up by aiming at clusters enemy players rather than a cluster of siege weapons. The priority targets are their siege weapons, not the players. (if you are an elite guild that lines up 8 cold fire ballistae and melts the players, please keep in mind this is a guide for beginners.) However, once you have halted their onslaught against your wall, you must understand the only way to truly end a siege is by charging them. As long as you don't take the ground they are on, they can keep resurrecting their players, so as defenders, as you beat off their attack, you are constantly watching for that moment where the attacking force is in such shambles that you can completely route them by simply jumping off the walls and thrashing them. This moment may arrive after you have crippled their siege weaponry, it may arrive when they have ten siege weapons still up, it may be that reinforcements came from another castle and attacked them from behind, but at that point your goal is to swiftly break the siege. As a beginner player, you probably won't be the one that intuitively feels the moment shift and then begins the charge. What is important, is that when you see it happening, you recognize it for what it is. When this moment comes, you must first recognize it for what it is, and then immediately do your part to make it succeed. There are two things that need to be done quickly, you be a big help in either way. First, the counter attack needs to succeed. If you are somebody able to contribute well in open field fighting, help the attack succeed. Second, the walls or door needs to be rapidly repaired. RAPIDLY. If enough players respond quickly the wall or door could be quickly repaired in sixty seconds. So even if the counter attack fails, it may have yet bought the reprieve that allows you to repair the walls and door. Not only does this help you defend again when the attackers start firing once more, it also can repair your keep enough for all dead players to respawn or reinforcements to travel to the keep. Meaning your companions who failed in their counter attack, can immediately rejoin the fight, and other defenders who have anxiously been eyeing that battle on the map can join in. But the cycle continues, until a counter attack works and you sweep the attackers.
Good luck. I hope this helps some of you enjoy Cyrodiil. I want to encourage you to try it, because if you use siege weapons properly, they truly are a wonderful way that new players can meaningfully contribute to PVP. And if you are each thinking in sense of this three step process, you will naturally coordinate in the best way possible given that you are only companions of circumstance rather than a pre-coordinated defense.