You will encounter two types of doors in the game: silver ones with red outlines and black ones that have yellow-orange flame patterns. The silver doors with the red outlines are safe and signify that a new area lies beyond. The doors with the flame patterns however indicate a boss fight is about to happen. Before engaging in this boss fight it is advised that you go back and save, check your equipment, and level up as much as you can. There are also certain doors in Bloodstained: Ritual of the Night that require keys to access them.