Reality Factory

Does anyone have any tips on making a level look organic (that is, the corridors aren't blocky) and still be easy on rendering?

There are many mapping/modelling techniques for that .. too many and too complicated to explain here and now. I suggest you google for tutorials and other documentation of the software you are using.

Use static meshes. BSP geometry cannot be concave. It must be convex. The best way to do this is to use the BSP to make the "shell" and use static meshes to detail it.

Bumpmaps will allow you to add texture and depth to the walls without much work.

...but only if there is light source.

One of the best ways to add atmosphere to a level (and then the palyer will not notice anymore that the corridors are blocky) is to add lights at the right positions. For what else do we have lightmapping?

Thank you for all of your help, it is most apriciated.