White-boxing is a design term for laying out ideas using placeholders, usually with the intent of replacing them with finished assets at a later date. In level design, the practice of white boxing is to block out an environment with primitive GameObjects to get a sense of how you want it to look. This is a great way to start things off, especially during the prototyping stages of your game.
Before diving into Unity, I'd like to start with a simple sketch of the basic layout and position of my level. This gives us a bit of direction and will help to get our environment laid out quicker. In the following drawing, you'll be able to see the arena I have in mind, with a raised platform in the middle that is accessible by ramps, complete with small turrets in each corner:
