Okay, very basic tutorial.

First, save map (with 1 side done) with another name (like sf_iceland_blueside). Then, delete things that were already made (middle wall, logic_gametimer). After that, group everything, Right click on it, select transform, Chosse rotate and in Z type 180. Tada! Also, you can rotate it w/o grouping, but you need to Configure hammer to 15° rotations (Tools > Options.. > 2D Views). After done all this, change team specific textures ( Texture Application(Shift + A) > Replace and choose firstly texture you want to change and second thexture you want to change to(like first redmetal1 and second bluemetal1).

1 thing left. Entities. You would need to change it 1 by 1 (if you don't use prefixes). But, if you are starting to make mirrorline map, go ahead. At every Red (or team you are firstly mapping for) entity, use prefix (name before unique entity name, like train, spawn_counter, etc.) that would be like redteam_ (so the entities would become redteam_train, redteam_spawn_counter).
Time to change to other team? Open the .vmf file in NOTEPAD or any text editing program (Crimson editor, Notetab, etc)(NO, word is not good, wordpad yes). Use replace tool (in notepad: Edit > Replace) and replace redteam_ with blueteam_. Then save, copy blue "map" into map with red (be carefully how you align pasted brushes! If you are using dispacements, and things dont fit, select opposite faces (or just all, to be sure) and use Sew tool in Dispacements tab).

Goddamit, i have too much time :shock:

__________________
ONEPOINTTHREEPOINTTWO

This is a good strategy, but this is actually symmetry and not perfect mirrorline. If you really wanted mirrorline, you could duplicated everything, then line it all up to exactly the same place as the already existing geometry. AT this point you may have to change to rotation mode (just click inside the selection square (when you have the move cursor) in the 2d views. The corner squares will change to circles and the side squares will disappear). After that, go to tools -> flip -> horizontally. voila! I use this all the time btw, it's very handy.

most of the time you will be using murkrow's strategy tho, as you see more bilateral symmetry than you do mirrorlines. The flip just comes in handy for small details inside your level, not so much the entire thing.

i really wish i knew that earlier/understood all of that.

heh hang in there chaos.