Most of the economic/"peaceful" cities also have invasions that you need to deal with, though they're smaller than if you had chosen the military city.
The save format was reverse engineered. Quite a bit was already known from my previous tool to create a .png image out of a saved game file, and from someone who created a Mac <--> Windows converter for saved games: the Mac version of the game used a different file format. Julius also contains tests to make sure it stays compatible.
Not specifically C, but you do want to have a language that's as close to the hardware as possible. Since Julius supports way higher resolutions than the original, the time required to draw the screen also increases: full-HD resolution requires 6.75 times more pixels to be drawn than 640x480. I've had to make optimizations to the drawing code to get a decent framerate on higher resolutions.