The "world" is our solarsystem over two billion years in the past. Venus, Mars and several other planets are full of live and Earth has produced some live, but not much. Atlantis is a domed city in a landscape that currently is covered with several miles of ice (we call it snowball earth period). This snowball effect is a sideeffect of the Atlantian process of creating an breathable atmosphere.
Intergalactic travel exists, gates, portals and high tech spaceships exist. Atlantis also has a gateway connection with Saturn.
Venus and Mars are in a cold political war fighting over Jupiter and the promise of resources. The council on Saturn (with a big military influence) has announced that the rights on the planet will be lifted in the nearby future, effectively meaning that claims can be made to harvest the resources. This is done under galactic rule.
Atlantis hosts the travelers that come by spaceship and is very well known throughout the (known) galaxy for its technology. The technology is very well guarded and not available for travelers. This is to protect Atlantis and the political status as is.