How to get started with level creation/ atmosphere?


I’m fairly new to Unreal, modeling, and everything related to video game making.

I’m developping a narrative detective game, and my first level is a big old mansion. It’s 3D and with a top-down view (point & click). I have all my mechanics set up and working (almost), and I made a first level for this house, but I don’t really like it, it lacks something… original.

So, this is a very large question, but how do you make an entertaining level? How do you set up an atmosphere in your game?
Do you know any “workaround” that can get your game an original and quick style (e.g. cell shading, black and white…)?

Also, I made the rooms of my house normally sized but I feel like they are actually small for playing, is it common to make rooms bigger than what they would be in reality?

Players might stay in this house for an hour, so I want it to feel enjoyable and special, but I don’t know where to start!