Level Design advice.

[Damascus Rose]

Nights should have a faint blueish light made with a boxlight to simulate the moon. And of course get a nighttime skybox. Turn grid snapping on when placing walls and structures and furniture, turn it off when adding specific rotations and details (normally for smaller entities, but it depends what you are trying to achieve.) Remember to add particle effects (like dust in windows, fog in some areas), billboards when necessary (windows), and decals to your environments. They really make the environment look at lot better. It will take a while until you get good at adding these artistically, don't just stick them all over the place and expect it to look good. Also turn off the alpha for most point lights, and box lights. To be honest I'm not entirely sure what alpha is, but I think it enables certain effects on the lights which will just lag the game when they aren't being used.

Don't jam a bunch of monsters everywhere, it will get annoying. For each monster should be unique and scary. Create environments where the player may think a monster may spawn- and leave it that way, then when you add an encounter it will be scarier.