With the Beggar's aid and the chimney's blocked, things are very well prepared for you to start into the Hideout that is in the Catacombs below. Head for the chapel and then go along the eastern side of it to the small yard behind it. 2 Extremists will be complaining about the smell of things. Take them out or avoid them then head into the entrance of the catacombs in front of you.
Head down the stairs. At the base you have 3 Extremists watching the area. Take your time to kill each of them in turn. Hitting one with a Berserk blade can be fun as well, but it is a very good idea to save those. Once they are dealt with, head over to the planks on the floor. Interact with them to get into the Hideout you are looking for.