Anyone remember the original Stronghold from FireFly Studios? Well, there's an open source remake named Stone Kingdoms. Currently in-development and available on itch.io and GitLab, the code is under the Apache 2.0 License but it also includes the artwork from the original game with permission from FireFly too (according to the developer of the project).
Developed with LÖVE, a framework for making 2D games with Lua.
Right now it seems it needs some more developers to jump in and help, with plenty missing from it, so it's not complete and playable. However, it does have enough done that you can load into the game into a map, and start building things so it's clearly off to a great start.
Looking forward to following this along, Stronghold would definitely be a fun game to have a complete open source game engine replacement for that would allow it to continue living on modern platforms long into the future.