Breakthrough is the award-winning crown achievement by Dan Troyka. The trivial rules generate a dazzling emergent complexity, studied and envied by many!
It is rather similar to Ugly Duck, also by Troyka, but lacks the promotion and race back home.
Thanks to its fame there is a fair amount of interesting writing to find about Breakthrough. For example it was (briefly) covered in Addison's book 100 other games to play on a chessboard.
Chessicals also implements the Siege variant invented by Chris Huntoon.