Level design
Here you’ll learn: What is level design?
Under level design (often referred to as “mapping” or “editing”) refert to the creation and editing of 2- or 3-dimensional levels and game worlds for computer games.
Levels are created with a program which is calles level editor.
In my case it’s the Hammer Wold Editor by Valve.

The tasks of a Level Designer be to find an appropriate balance between an architectural design, the gameplay and the performance.
Depending on the game engine of the level designer, a quality sound effects and the required lighting for the Level add and customize.
At levels for a single player mode, the level designer creates environment-based puzzles and placed opponents or monsters and where appropriate NPCs.