Field of Glory II: Medieval offers very many modding opportunities. You can create new scenarios, alter unit tables, modify the campaign structure, and adjust the game rules and user interface.
For help modding and creating scenario scripts see the modding section of the ARCHON wiki at http://archonwiki.slitherine.com/index.php/Modding
Also please visit the Field of Glory II: Medieval modding forum at http://www.slitherine.com/forum/
The recommended modding system works without making any alterations to your main game build. You are strongly advised not to mod your main game build, especially if you intend to play MP games, as these will not work correctly unless both players are using exactly the same game scripts and data set.