No expert but anyhow.....
1\ Your diffuse.dds is basically your textures.
2\ Your normal.dds is a method for giving the appearance of a structure when there is none actually on the mesh, the appearance of more 3D detail.
Below I have generated a new normal map for the Roman soldier (will explain how later) As you can see I have simplified the map by removing all the other detail because I wish to only show the cross as an example.
The normal map now creates the appearance of a cross on the Roman shield even though there is none on the mesh.
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Normal maps are generated within a 3D program from the model, what game developers can do is create a high polygon model with a lot of detail create a normal map and use this to give the appearance of more detail on a low polygon model for a game.
We do not have access to the models but you can also create a normal map from an image.
The simpler the better for the cross I created an image like this, black , white or greys seems to be the best for clarity.
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In Adobe go to filter then in the drop down menu go to Nvidia tools \ Normalmap filter
Here you can adjust settings etc and generate a normal map. You could then copy and paste that into your Roman normal map where the shield is and tidy up.
3\ Your Specular.dds is a method for telling where a texture should shine and in what color.
Not sure how it works in Rome but certain colors will translate into different colors on the model. Green turns objects red. You could turn the whole specular .dds black and all the textures from the Diffuse file will show, but because its also used in Rome to give a different color for the rebels by reversing or changing the instructions for what a color will do you better not.
I think you could ignore it unless it affects a color that you want, for example if you want a blue shirt and the specular.dds keeps turning it red your going to have to change it.
Final note I am not an expert so be careful what you take from this, nothing will blowup and it should help but more research is needed.
Edit: by the way the Wreath was not finalized because I could never get Rome to do what it was supposed to do with the Blender files. It would put it in the game but use the wrong textures.
Could never figure out why and then I stopped playing Rome.