Yea....
But I'd really like a gunpowder specific defense.
Right now the closest thing to WWI style ridiculously effective defenses are machineguns (before tanks, aircraft, and mechanized infantry)
IRL trenches are great against advancing infantry and artillery, but not; tanks, helicopters, and aircraft.
I guess what would be really cool is the ability to have your workers build "trench lines" that act like rivers, attacking across them incurs a 25% penalty (ideally this wouldn't apply when you are attacking within your cultural boundaries...)
Maybe with python there could be a "moat" building, once built, a river rings your city.
Amphibious is not *that* common, if it came with a -2 health to offset the freshwater boost (assuming your city didn't already have fresh water).
But then there is the issue of using it to allow hydro power plants, and getting bonus production with levees/dikes