Level Entrances
Every map's entrance is at the origin of the map. You can locate this in Hammer by looking for where three coloured lines join, or in the 2D views, where the axes join (the dark green lines).
To see how the previous level's exit door will interact with your map, copy the 'level_end' entity from your map and move it so that the 'X' is on the origin (where all three axes cross). This is where the door will be after the player enters your map. Remember to delete this 'level_end' once you have found the shape of your entrance!