Starting details? As someone who actually doesn't know anything about the content you speak of, Mr. the Cat, I admit that my suggestion might seem far fetched or obvious.
It sounds like you've got the perfect setting for a number of different stories.
There is the Gate cities, which could face problems to foreigners and people traveling to them for the first time that would seem really weird. The monsters we can't dream of for instance could be something they had to deal with daily, creating a need for adventurer's to go out and strike back while also allowing them to have some adventure's within the cities as well.
It could be opposite though. These gate cities are very good at staying out of the mysterious lands beyond their gates because of the dangers involved. Everyone knows there's some ancient riches to behold but the myths and legends of the land have become a cultural inheritance, passed down and growing from one generation to the next, each story emphasizing how dangerous it is. Right up until one slightly crazed merchant with a -lot- of wealth and some 'secret intel' about the location of this lost city or that lost artifact comes around and starts an expedition. A very well funded one, not for the faint of heart, allowing for a more... savage game where it's about the mysteries of the land?
It could be a combination, starting with the former, a defense of the city against some of the dangers, and slowly leading to the latter. The city has been defended and the locals are much safer then they have been for generations thanks to the proud heroes. But now one local in particular wants to venture forth on an expedition?
Again, these are ideas I had without researching the setting at all, so they might not fit in the least. If that's the case, I do apologize for wasting your time Mr. the Cat.