Hi everyone!
First of all, let me apologize for the question, which I know is quite silly. But I’m a newbie with Unity and just started to move my first steps.
I would like to create a simple FPS environment. I’m thinking rectangular rooms, with just walls and doors (at least for now 
). Think Wolfenstein. Maybe think Diablo.
How can I proceed for the best results in terms of performance and aesthetics?
I’m tempted to make a generic wall tile and then to clone it all around building rooms. But is this good performance-wise? Am I adding too many vertices to the scene?
Or have I to build and skin the whole level in (say) blender and then import it into Unity as a single asset?
Thank you!
You have absolutely all the posibilities you’ve just mentioned plus more.
Creating art for games has endless things to consider and tweak. Don’t be afraid to try things out, though. Do it your way first and see what’s going well and what’s not. In the end there is no “right” or “wrong” way as long as the aesthetics fit your concept and the performance’s right.
The best way to get you started in what is possible is to have a look at the Unity example projects.
Another interesting idea for (albeit high-end) FPSs is to install UDK and have a look at the modularity in their maps. Amnesia - The dark descend is also a game that is heavily tile based, for example. If you have that one - download the editing tools and look at their approach to things.
Another good place to search for knowledge is the Polycount Wiki (http://wiki.polycount.com/Polycount). There’s TONS of very good information to find there. In fact so much you might get lost at first ^^
As a complete newbie I’d suggest trying to first model a room or two in your 3D program. Lay out those rooms to your liking and texture them. After that you create single objects to decorate your rooms with so you can improve iterative. Create those things as single objects so you can place them as prefabs. For one it’s better for memory usage. Then it’s also fgood in a way that if you change a mesh all the meshes you’ve placed change. Not only one.
Hope that helped to get you started 
Motionblur it’s alright. Check him recommendations, I personally like the tile based games, you can make new levels very fast.
You can use also Deled3D editor it’s a very simple low poly level editor with texturing options and animation for doors, windows, mechs, etc.
it’s completely free and the exported levels run very well on Unity3D
I’m afraid I’m working on a Mac.
Beside this, thank you for the precious advice! I will work with blender on this one, it seems to me a rather good tool for the job
I’ll keep you updated on my progress
Good luck!