I had so many problems trying to do this on my level 59 guardian. Thought it was a level issue and pushed to 65 and tried again. Same prob in the courtyard. Mobs kept swarming around and stomping my skinny little kitten into the ground. I found a really simple solution to this after about 20 deaths. So simple it will make you want to puke…
the first siege item right down from Trahearne with the the charr chick, it’s a gigantic ghost cannon, after its working again and you’ve cleared the other 2 siege points, go back and use it to blow the hell out of all the risen in the courtyard. I cleared it in about 2 minutes then went down and the red ring turned blue about 30 seconds later. You just need to make sure you kill all the veterans or the trash mobs will keep spawning.
Edit: I’m new to GW2 and I’m not sure if the spoiler above will apply with all story choices, as it might depend on what npc’s you have chosen to accompany you.