Go in through the main entrance of doomsday, walk along the road, find that you can enter the slum, and then go straight ahead. There is a place opposite the square that needs a crescent key, which is Doomsday North (the library is in Doomsday North). When you go in, you will find that you are in a monastery. Avoid the guards patrolling the road, jump to the door below and go straight ahead. I'm not sure. You'd better find someone to brush it alone and take you down.