The portal back to the Nave is on the ledge behind you and can be reached by jumping up the steps.
Once the battle is won, enter the small door in front of you. Though you can check through the cage around the left side of the door to see the Glaive Seal (fighter weapon icon) you are after if you so wish.
Cross the metal bridge over the small drop, but watch for the Slaughtaurs on the raised alcoves on either side and open the door to enter the heart of the watch where a large group of Afrits greet you.
Take a sharp left from the door you entered from and then another left through a small wooden door behind which is a corridor of water with a switch at the end. This opens up bars at the top of the small wooden stairs you likely noticed in front of you when you entered the area. Go up those stairs and press the switch up there to open the large doors to the right of said stairs.
Inside are more Afrits. In the top left corner of this room is a raised area you can jump into. In this small room in amongst some barrels is the map's gear.
Go back to the Nave to place it.
Return to Dark Watch
Once you have placed the gear, a message will tell you that some stairs have been built on the Dark Watch.
The stairs in question are what was a ledge along the right side of the main area of the map. Head up them into a room ringed by stained glass windows. Smash the one which is rightmost as you face them (if it hasn't already been smashed by stray fire) to reveal a door behind it. Down a corridor behind it will lead you up to a narrow area that you could see through windows from the outside. At one end of this area is a switch that opens some bars blocking a teleporter.
The teleporter can be found down the corridor immediately to the right of where you first entered the map's main area. (just to the left of the stairs you lowered with the gear). At the end of said corridor is a room with a Maulotaur statue on a raised platform. In one of the corners of this room is the now-accessible teleporter.
This will take you to the cage containing the Quietus icon that you could see from the start of the map. There is a portal in here that will take you to the start of the Cloaca. Once there press the switch in front of you to return to the Nave.