Originally Posted by TheGamingFilly
There are some areas I simply cannot find (e.g. Underworld: City of the Dead) through trial and error, which is incredibly frustrating.
City of the Dead is easy to locate. It's Mark #15, which is right after Mark #14... It's pretty easy to assume that the Underground River should lead to City of the Dead, then. Same basic techniques can be used to understand the other maps.
Edit: Saw your reply to me. It sounds like you are relying too much on the maps (made in order of acquisition) to determine your location, which is what's getting you turned around. You may want to simply close the guide after filling out the sort of information I offered above... or at least give your brain a rest. I could never acquire achievements very well when I was flustered with them.