Unpacking Medieval II: Total War Files
A Pack Extractor is needed to open or change any files in M2TW. The pack extractor was not included in the initial release of the game. Community attempts to create one failed as the files are just not packed, they are also encrypted. CA says this helps game performance.
Creative Assembly included a pack extractor with their first patch (v1.1) which can be found in tools/unpacker after the patch has been installed.
Even after unpacking some files are still not immediately editable. To edit the contents which make up the strings.bin files in data/text folder you need to use Alpaca's.
 Unpacking Rome: Total War Files
Some of the unit and graphics files in RTW are contained in .pak files, see Unpaking Files:Step-by-Step for details of how to unpack for that game.