To expand a little on some of the rationale:
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General Theory of Tower/Keep Defense
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There are three primary ways to defend a fortification from a larger force. (I won’t go into defending a fortification from a smaller force – just jump down and run ’em off).
1) Delay and Pray
Use your resources to hold the opposing force off as long as possible while calling for backup. By holding up large groups of people attacking your fortification, you free up men on your side to take enemy objectives as well as reinforce you, possibly flanking their assault force. Still, though, that force isn’t always there…
2) Porcupine
Have a defense that throws out so much damage nobody really wants to approach it, “suggesting” they go attack somewhere else. However, that somewhere else is probably going to be a supply camp or two, and sometimes they just really want your fortification or don’t have any other choice.
3) Kill them.
And not only kill them – kill them in such a way they can’t be revived. This is the hardest thing to do, and it’s not going to happen on the ramparts – they can spread out so far, and there are all sorts of abilities that will let a group run in and rapidly pick somebody up. To kill them, you’re going to need to knock a whole bunch of them out at once, and then make the area where they fell so impossibly dangerous that nobody can approach.
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Ramparts
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1) It’s a murder zone for the defenders –
When you’re on the ramparts, you’re constantly getting bombarded by AoE, scorpion wires, and other mechanics that have a chance of forcing you off those walls and onto the ground, where the enemy team will savage you. Now you’ve gotta run back, because nobody is going down there to pick you up.
2) Attrition -
You can hit them with siege equipment, you can hit them with AoE, but the attackers, more often than not, can use some fancy skills and get themselves revived. Every time they knock somebody off the walls, you’re not coming back. Attackers will generally win the battle of attrition.
3) Siege on the ramparts –
I see lots of people putting arrow carts, ballistas, catapults on the walls themselves. Let me tell you – I play a staff elementalist with upgraded AoE size, and we can probably break anything we can see. There are probably only a few places per tower/keep to put siege equipment right now where it can’t be hit by determined players, but this doesn’t really matter because…
4) They will break in. Eventually. (Most of the time.) no matter what you do, they’re going to crack that door. The only reason they’re going to stop is if an allied force can rush in and let you sally forth and take the fight to them, but quite often that force just isn’t available.
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Defending the door
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1) Surprise –
Those walls are tall, and in most places the attackers can’t really get to a high enough place to see what’s inside. That means if you build something in your courtyard, on the ground, they’re probably going to find out about it when you introduce it to them personally. Right now (and this will probably change eventually) as a gate goes down people just pour into the doorway – you might have 30-50 people all smashed into an area that’s about 20′×20′. If you can show them a catapult, a few arrow carts and maybe a ballista, you might be able to down 80% of them in a few seconds. I’ve engineered that scenario more than once.
2) Delaying foes –
By now I’m fairly certain everybody knows that AoE effects will go through doors. This works especially well with catapults inside your keep/tower pointed at the door. By lobbing rocks, you can knock down people on the gate, buying you time for an allied force to get there. Worst case, if they crack the door open, you’ve still got a catapult pointed in the right direction.