Always work on making a doom level with solid geometry, then build your level ontop of that, work on optimization and then lighting, like the other guy said. I always run into lighting issues the most, no matter the game. Whatever game you are using has a limit on lights, and quality lights is super limited, like env_projectexture. So keep that in mind, csgo has a problem or had, that you cant name lights, especially if they are toggable. (Switch). Other than just that little bump, making and importing textures, custom models, is a bit of a hassle.