1. The outside world is reasonably dangerous. Put simply, imagine if you were adventuring in the Pokémon world where there was twice as much tall grass and a good chunk of the Pokémon would actively attempt to kill and eat you. People are allowed out of Union, but it's usually under guard of someone capable. If you wanted to go yourself but were not a trainer or Enlightened people would definitely try to STOP you but you could do it.

The Old City ruins are commonly entered, usually by the Delvers who are looking for old artifacts for salvage. There aren't rules governing it per se, but it's kind of like the wilderness, there's a lot of stuff that will try to kill and eat you you down there so they suggest you only go if you're prepared.