@Mr_M0ntana - Thanks for the info!
@DVS - The 'designs' are in the game and used like normal resources. As tron47 proposed, a civ-specific wonder provides a limited number of designs to each civ. They need that resource to build all of their units, as well as any other prerequisite resources (like oil, iron, coal, etc). They can trade it if they want, but then that civ could build all of their units.
The only way B&I restricts this is that all unique units are represented by their own unit class - that allowed me to restrict given civs from building those units in CIV4CivilizationInfos from building certain units no matter what. That means that if for some reason Russia were to capitulate to early to Germany, there's no way that Germany can build any Russian units.
@tron47 - Right now the only defensive improvements are forts and trenches (though trenches haven't been implemented in the XML yet). Different levels of trench build-up will represented graphically, but will have no different bonus. I wanted to keep B&I relatively simple from a combat perspective, so you don't have to worry about entanglements.
I haven't thought about fort attack. It's something to think about, but for now I don't think I'll add it. The alpha is getting close to release (maybe a month or 2), and I don't want to slow it up.