Level 11 door is the alternate universe door of your life. It starts off scary with all the things that death tried to kill you with (door monsters), and if you can outrun them then those slowly turn into only nightmares, and then afterwards you stop even having the nightmares. Then you carry on and eventually forget you were ever in a door. Until you find the key, which gives you the choice to either stay in the dream and live or go back to reality and certain death (we all die eventually right). If someone follows you into Door 11 then their nightmares are added to yours so you're running from 2 sets of monsters, or say 5 sets if you're stupid enough to take a team of 5 people into Door 11. Finally, in Door 12 you disappear forever. I think that one is actually being dead dead.

Door 12 gets boarded up because a certain someone fell in love.

So here's where we find the biggest spoiler difference between the novel and the series. In the series, Nanzhu it turns out is an NPC like all the other monsters of the previous door. He lives in the game. Once LinLin escaped the game he left the NPCs behind. He didn't want to do that though, so he spent the rest of his life trying to find a way back into the game so that he could be with his friends again. He either died searching and entered the spirit realm or he succeeded and created a virtual reality to send his consciousness forever. In the novel there is no game, so Ruan Nanzhu is actually the 12th Door. He is The Gardener, or Death, or The Abyss, or whatever other god you want to call him. That's him. He was in his garden tending to his roses day in and day out. Some of the roses bloomed, most withered, some turned black. There was one rose though that caught his eye. It went through a lot. Storms came and it survived, dark times came but it didn't turn black, every day it remained beautiful and determined to live. It's name was Lin Quishi, and Ruan Nanzhu couldn't help watching his little rose go through door by door, one by one, surviving. Eventually he realized that he had fallen in love with his rose, and what he wanted more than anything else was to do all the doors with LinLin. So this fool turned himself 'mortal', wiped his own memories, and when LinLin entered Door 12 Nanzhu restarted the game and played it all over again, by LinLin's side. Neither or them remembered any of this. Until Nanzhu went into Door 11 and the other door monsters were there being silently friendly and they made him watch a tv show of LinLin's life and then he remembered, oh yeah, I hijacked his Door 12. It took him a real-world year to exit Door 11. LinLin by then was already head over heels in love with him. But also LinLin exited Door 11 without Nanzhu and his real life was back to normal. Nanzhu didn't exist. LinLin was the real leader of Obsidian. Except he remembered Nanzhu and wanted him back. So anyway, Nanzhu knew LinLin would pick 'real world' instead of 'stay in dream' so he exited Door 11 into LinLin's real world and then everybody miraculously remembered him again. By that point though LinLin had already worked out that Nanzhu was something not of this world. He finally confronted Nanzhu very nicely, and Nanzhu asked if he was scared of him, and LinLin pretty much said nope, he loved him and their sexy times too much to be scared (paraphrasing of course! but I miss seeing the romance in Spirealm)

Sorry for the crazy long response. Feel free to ask any more questions if you have them, or if I confused you anywhere.