This version is really not a lot different from RS2.6 in terms of how the game will play, units will fight, etc. Stats have been tweaked a little, as well as unit prices and economics in general. But by and large, that aspect of the game has changed very little. Significant changes that might cause some discussion were made to all siege and mechanical weapons, however. The other significant change is to general units. All of them now have the same number of units as a 'regular' cavalry unit would have. This was to encourage the AI to recruit them, and since all generals are effected, the impact on battles is negligible. To the full list of changes you can find link in description.
This is a stand-alone installer, so I'm recommending the following procedure to install this mod:
1. Create a new, clean install of RTW (with or without BI) which of course is patched to RTW version 1.5/1.6.
2. Install RSIII using this installer, it's a simple 'point and click' (just point it to wherever you installed RTW).
3. Use the launcher to play.
!!! Choose correctly in launcher preferences the 'descr_formation' settings corresponding to .exe file you are using (see fig. below).
!!! Be sure and setup correctly in launcher preferences the screen resolution of RTW (both strat and battle) to whatever the default resolution is of your system. The game will crash if you don't (see fig. below).
If you don't like the newer stats in RSIII, the complete set of RS2.6 stats are available in the 'data\_Important_Stuff' folder.
STEAM USERS ONLY:
RSIII will NOT work at all with the Steam versions of RTW or BI. So, if you must play RSIII in Steam, you will need a copy of the RTW Alexander expansion. Recommended installation:
1. Copy the contents of the Rome Total War Alexander folder to another folder, ie., 'C:\games\RSIII' or something like that.
2. Rename the 'Alexander' folder 'No_Alexander' or something similar, if you want to play Custom Battles.
3. Then install RSIII into this folder.
4. Use the launcher to play.
To the full list of changes: twcenter.net