johnpjones1775 wrote:
1. So the empire is based on the Holy Roman Empire from about the 16th century....
...
2. I guess the simple explanation is rule of cool.
Your answers
1) Sort of. It's better not to try and link any Warhammer unit/faction/etc to any specific historical setting. It's even sillier to try and justify any aspect of Warhammer based on it's effectiveness in real-world historical settings.
As for the preference for medieval fortifications, I can only speculate that it looks good, and a square castle is much easier to scratchbuild and to produce as a plastic kit.
2) You answered your own question.
I respect your historical knowledge, but you'll probably be more content if you consider actual history as source of minis (you can do a pretty sweet empire or brettonnian force via historical minis), transferable stories and paint schemes rather than a template for accuracy.