Could someone tell me how to mod structures in for the different factions?

Basically, what I want to do is make some of the more advanced fortifications and roads available to the barbarian factions. How would I go about doing that?

You'd edit the export_descr_buildings.txt file in your /data dir so that it shows the word "barbarian" (or any faction you wish) between the { and } brackets wherever you find the term "requires factions" of the buildings you want to enable.

This creates problems that I think are hard coded. Unless I change the requirements for all factions. Like Highways only available for Huge cities. Plus, the icon and description doesn't show up properly when using this method.

Any suggestions? ~:)

My suggestion:

Using a search function, find all files in the Data and subdirectories that contain the reference for a particular building you want to make available. Go through each instance and record what the links are to other files, such as graphics, and the faction it currently exists in. You will then know exactly what you have to do to add it to the new faction, just create new entries and links exactly as you find them but with the new faction.

Be forewarned, many links exist so you will be spending a lot of time tracking down all the little things you will have to change to make it happen.