A brief summary of Twilight Forest:

Wizard Tower, Druid Ruins, Hollow Hills(filled with riches beyond your wildest dreams), Hedge Mazes, an Array of new mobs: Rams, Deer, Squirrels, Birds, Dark Wizards, Nagas, New breeds of Spiders, Dwarves, And much much more!

http://www.minecraftforum.net/topic/561673-132-the-twilight-forest-v1113-updated-for-132-bug-fixes/

This is a tutorial on how to install the Twilight Forest for 1.2.3:

1.You will need an Archiving Program like WinRAR or 7zip.

2. You will need to download Minecraft Forge from this link: http://jenkins.minecraftforge.net:7070/job/forge/242/
-Download the Universal Build.

3. You will need to download Twilight Forest(obviously) from this link: http://www.minecraftforum.net/topic/561673-132-the-twilight-forest-v1113-updated-for-132-bug-fixes/

4. (This is where the archiving program comes in) You should open the Minecraft Forge folder and open the minecraft .jar folder it can be located here in windows explorer: C:UsersOwnerAppDataRoaming.minecraftbin

5. When you have opened the two folders you need to drag all the contents of forge into the minecraft .jar

6. Delete the META-inf in the minecraft .jar

7. Finally, create a new folder in your .minecraft folder( C:UsersOwnerAppDataRoaming.minecraft) called mods then drag twilight forest into it!!!

8. Enjoy Twilight Forest!!!!