Though still in the beta stages, the Assembly Kit is a full-featured suite of tools useful in modding the historical strategy game. According to publisher Sega, the Kit includes everything necessary to modify the game's existing assets, as well as import/export tools for pulling artwork, audio and character models into the game from third-party programs. The Kit also includes a swath of "example data" which should be useful for nascent modders.
The best news, however, is that the Assembly Kit is available free of charge to anyone with a copy of Total War: Rome 2 and an Internet connection. Full details on the Kit and information on how to download the tools can be found on the Total War Wiki.