After beating the story you can use fast travel to go back to all the main areas in the game (Slums, Old Town, Antenna). This is very important for searching collectibles and completing all side quests! The game doesn't actually tell you that you can go back to the other maps or that there is a fast travel option. To fast travel between the Slums and Old Town you need to head to your bedroom in the tower. There's a poster on the wall that lets you travel to the other map immediately. To get to the Antenna area you need to use the door in the north of the Slums or the sewer entrance in the north of Old Town.