Just to throw my opinion in, I think Besieged will be a lot better in weeks maybe months to come... it wasn't designed for nearly the amount of people who participated, just as the dunes weren't designed for 100 Corsairs, 200 Blue Mages and 150 Puppetmasters (Exaggerated of course). Everyone just rushed to try it out...
As far as the actual event itself happening, besieged is made to be done in the city. It's right on the developer info page, and it's all through the CS's in game. However, the ability to intercept mobs before they get to the city was added in, so, think of this with normal conditions, not 400+ players. Having an alliance intercept some of the mobs to weaken the main attack force might be a good thing if there's only, say, 36 people participating.
But, as I've been reading on the posts here, I don't agree with what whoever did by preemptively trying to defeat the beastmen army. The only thing they wanted was glory and bragging rights, which I can understand, because what big LS wouldn't want bragging rights? Especially when they have their rivals waiting around in the city for the attack to happen...
The point stands though;
Besieged (click for definition of the word) is an inner defence of the city, if people want to try it out, people shouldn't ruin it for them. If you want to compare this to dynamis... it's a role reversal. We are defending our city, and they are attacking. But, I don't see beastmen invading the main cities from the dynamis areas, because that's not what the developers intended. This isn't a real life war situation, it's a game. If this was a real life war, I can understand a preemptive attack on their forces, but this isn't... we're playing a game, trying to experience a new event. If you want to develop strategy later, wait till everyone can try it out first, for fun, if not anything else.
Summary: Beastmen want the Astral Candescence. We have it. They come fight us, we kill them, and keep it. If they take it, we go kill them and take it back. You get bonus's for defeating them inside the city, instead of outside. Doing otherwise is like using an exp bonus ring to fight too weak mobs, it's a waste when you can get *something* for it, even if it's a small amount. However a preemptive attack to weaken the forces with a small amount of participating players could be a good idea, but when it's clearly able to be taken in the city with an outragous amount of people participating, and seeing you get bonus points, anyone who argues that point is clearly just trying to defend their rights to glory and bragging rights by killing them before anyone else gets a shot, that or their pride on the FB...