I have finished changes on HRE too now. You can build these cool buildings and bridges from HRE Siege Engineers (HRE 2nd Age Archer Unit) from Capital Building by recruiting the HRE Villager.
And I have added stone bridge option after a long testing. For some reason, the Eastern Bridge turned into Red-Box Pre-Cache Visual Error, so I use West bridges. I will add this Bridge option to other remaining civilizations later on; because it takes lots of time.
