The OpenTomb team wasn't able to retrieve the original Tomb Raider source code from Square Enix, so developers simply decided to re-make the game from the ground-up. They built their own engine and wrote their own code designed to take advantage of modern CPUs, graphics cards and gameplay features.
This open-source, from-scratch model sets OpenTomb apart from existing Tomb Raider mods and patchers that clone the original game engine.
"The older [an] engine gets, less chance it'll become compatible with further systems; but in [the] case of OpenTomb, you can port it to any platform you wish," the developers write on GitHub. Their post continues, "No matter how advanced your patcher is, you are limited by original binary -- no new features, no graphic enhancements, no new structures and functions. You are not that limited with [an] open-source engine."
OpenTomb is an ambitious project and so far the City of Vilcabamba is its only result, but it's an encouraging start. The team is active on the Tomb Raider forums, if anyone is interested in following Lara Croft on this brand new, yet shockingly familiar, adventure.