Cool. His work is good, show more!!!
Do not hurry, you can be inspired (listening 10 hours of Epic and Inspirational music in youtube is sufficient). Use some reference, such as a tomb or dungeon image, (there are very cool concepts and wallpapers in pinterest, google images and deviantart). You will thoroughly enjoy and travel to the inside of them. Play games like Skyrin or Oblivion, If you become tired, watch a movie.
Before creating a level for Unreal:
First: Block all playable area using BSP keeping a gameplay in your mind (Anything cool, challenging or able to engage the player into something). Secondly: Export all the static meshes to 3d software package to add fine details, chamfers (pay attention to the small details and chamfer the edges), unwrap your meshes, texturize using seamless tiling, finally create the lod level and modularity breaking all into parts. Thirdly import all into Unreal 4, then work in the materials, illumination, prototyping and post processing.
Pay close attention over details, seams (texture and meshes). Do not leave seam, repetitive seamless textures and repetitive things apparent. Repetitive things are able to destroying any good work.
Use varied things like particles, static meshes, debris, torches, illuminated crystals, flickering candle lights, suspended dust and fixtures, spider webs, animated chains, doors, grills, skulls, broken chariot wheels, vessels, statuary (gargoyles?), medieval torture machines, etc. Whatever your imagination wants!!!
Set up the atmosphere and lighting. Use point lights, ambient light and skylight for a better setting (expressing , athmosphere, emotion and mystery). Equalize all using a very weak colored ambient light (Avoid the pure darkness). Use weak ambient light, fog, haze, various post processing volumes: using DOF, Vignette, Color Grading, Color Correction, Bloom, etc.
pay close attention to Specularity, intensifying humidity. You can to use panner over material simulating water dripping down in the wall and pillars.
See something about PBR
Avoid pure black
Avoid very simple geometrical forms
Avoid repetitive seamless textures
Pay attention to small details
Add more importance to scenes (points of interest).
Add more debris
Increase the visual field in sample images for everyone to be able to see a more variety of details and ambients in your scenes.
And more important. Do not worry about non constructive criticism, not pay attention.
Do not give up. And always do what your heart desires. If you are not happy with the appearence, return unafraid. Go back again, again, again and see how you can improve it and be happy.