Whether you are creating a game for a game jam, team project, or whatever reason...it is best to create a prototype using shapes first to see if your game is playable.
After creating your prototype, start playtesting it to see if it has bugs, which can include your players, enemies, or game objects not acting the way they should.
Once your game is playable to your satisfaction, you can replace the shapes with real game assets!
In this video, I am playtesting a prototype game that I made with the Unity Game Engine. I found quite a few mistakes, which is good before I went any further. (This is a how-to-video)