Principia is, in its simplest form, a physics-based sandbox game. In Principia, you can build contraptions and simulate them in the physics simulation. This could be a mechanical contraption, an RC car, or a pinball game. Principia also contains a LuaScript object which allows you to write and create Lua programs that can interact with and affect the simulation.
Principia also features the Adventure game mode. In this game mode, you take control of a small robot which can do whatever you decide it to do. It can take part in a giant battle with enemy robots, or it can take control of the giant robot and wreak havoc on the world.
Principia was originally developed by the Swedish independent game company Bithack AB during 2013 and 2014. Sadly, the game was later abandoned due to Bithack not being able to finance the project anymore. It is however still loved by a small but passionate community that still keeps the game alive after all these years, including creating this site.
In August 2022, Principia's source code was finally released onto GitHub and is now being further developed as a community project. What the future holds for Principia is currently uncertain, but it is certainly bright...
Principia-web is an open source reimplementation of the official Principia community site, created by the community for the community. It is currently developed mostly by ROllerozxa, with contributions from the Principia community.
It is fully open source, and the GitHub repository of principia-web can be found here. Also see the credits page.