This is the place where we are still waiting for that woman to translate that scroll? Some maniac of an Orc sent my main character there for a book ages ago.
Take a follower with you, you climb up the mountain of rubble, use whirlwind sprint to get over to the platform where you left the room originally, that door is of course now closed, your follower will rush around the long way go through a locked door and should appear on the other side of the door. I cannot rightly remember if they opened the door automatically or if you had to ask them to, but you can get back in there.
I think the locations they choose for these things is random, if you save before asking for more to do, and do not like the location you got, you could probably reload your save and try again. I never bother doing that though. But it is something to keep in mind.
Just reading the name High Gate ruins again was enough to make me want to cry. I remember spending
hours in there, trying to figure out a way back into the second half of the throne room, when the Orc from the college sent me.
Thankfully using a follower to open the door is not very time consuming once you already know what you've got to do. Bad memories though, very bad memories.
It's extremely important to ask the follower to do something for you prior to climbing up the rubble in the first room by the way, or else it won't work, because once your follower has made his/her way up onto the platform too, he/she will tell you that what you're asking is impossible. So ask them first, and then head for the platform. Once you've reached the platform using whirlind sprint, quickly click on the chain behind the gate to tell your follower to pull it. If you aren't fast enough, your follower might end the conversation, so don't be discouraged if it doesn't work right way.
I had to ask mine 3 or 4 times, before he actually pulled the chain (he was just standing there, staring at me from behind the gate), but it works and is easier than using a plate or bucket to try and glitch your way in I think (I never really got the hang of doing that).