In case the launcher does not work, and for some reasons we didn't manage to help you resolve the issue, please follow those instructions.
Running RSII through Steam is a hassle but not impossible. Several Beta Testers have successfully ran the game through Steam and have imparted their knowledge to the rest of the team so that Steam users are able to enjoy the game as well. To run RSII on Steam, it involves adding some extra lines of code to your game library and a bit off a different installation.
First, make a folder on your desktop. Install RSII to that folder. Once that is done, drag or cut and past all the files in the folder to your C:\Program Files\Steam\SteamApps\common\rome total war gold or whatever other version of RTW you may have.
For those who have Vista or Windows 7, the directory should be:
C:\Program Files (x86)\Steam\SteamApps\common\rome total war gold or whatever other version of RTW you may have.
Once that is installed, open your steam launcher and browse to your game library. You should see RTW or BI or ALX depending on the version you have. For demonstration purposes, I am using Portal (the only steam game I have).
Right click on your RTW game and select Properties
You should be taken to a window like this, select Launch Options:
Once you have done that, type the following in the box like so:
Click OK - BE SURE TO INCLUDE THE SPACE before the "-mod"
This will allow you to play the ROMAN CAMPAIGN only. To play the OTHER campaigns, simply type in the other campaign names in place of Play_(campaign name here).
For simplicity's sake, I have enclosed ALL the campaigns that are available here in this post:
Simply copy the campaign that you wish to play and paste that into the Launch Options. REMEMBER TO INCLUDE THE SPACE BEFORE THE "-mod_play"
If for some reason you can only play custom battles have a read of the 7th post in this thread: here.
This solution will be part of the RS 2.2 patch.