I waited until I was lvl 36. Because it's multi-level and there aren't a lot of dots around you can just wait until you're feeling more confident. Same with Golem court tower.
As a level 16 I'm assuming? I wouldn't recommend undertaking the challenge of Sunken City unless you've friends or are teaming up. Soloing this instance will prove quite challenging at your level. Norton alone will have two minions with him making the battle a 3 V 1. Not quite a feasible match up for a Journeyman.
I completed it and I am level 16, granted I got killed a couple of time and had to keep returning plus I had a few spare crowns so purchased some Henchmen during battle to help me out, but stay persistent and you'll get there