This appears to be a huge effort. The "Augustus" engine, which shares a lot of code with "Julius", has all of the incompatible updates (roadblocks, zoom, changes to worker behavior, etc. etc.).

The compatible updates (compatibility with modern Windows, MP3 based sound support, etc. etc.) are all together in Julius... I guess for all the people who have 20+ year old save files they want to come back to.

Creator of Julius here.

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.

Save game compatibility would be very useful for testing if a bug was reproducible in the original game.

Indeed, what blows my mind is not just an excellent open-source re-implementation, it also is (or aims to be?) bug-for-bug accurate [1,2].

[1]: https://github.com/bvschaik/julius/wiki/Caesar-3-bugs

[2]: https://github.com/bvschaik/julius/issues?q=label%3A%22origi...

My guess is that save game compatibility is a great way to get people to try the new engine, and it was probably relatively easy to reverse engineer.

If only 27-year-old me had realized that 50-year-old me would have need of save files from a game I no longer wanted to play.

The regret and heartache of not saving every floppy, and not imaging them sooner.