In celebration of Serious Sam's 15th anniversary Croteam are releasing the source code for Serious Engine v1.10, an update version of the engine in which the first two Serious Sam games were made.

It is a very old engine by this point, so don't expect it to be as simple to use as Unity or Unreal Engine 4, but if you're in a tinkering mood then why not see what made some of the most classic PC FPS games tick. Here's the brief announcement message and download instructions:

"Historically, this version of Serious Engine is very important for Croteam and for me personally. I created several mods for Serious Sam back in the day, before even starting the work on the source code, and it was a great tool for learning.

And it’s even better today! Obviously, Serious Engine v1.10 won’t produce top-notch graphics, but the source code is very well commented, easy to modify, and there are lots of user generated mods out there. This version has everything you need to build your own game – or just experiment"

You can grab all of the engine files from GitHub, and if you need any help its recommended that you visit the Serious Zone, a community of Serious Sam modders and map makers.

Here's how it looks behind the scenes

According to some comments I've read you might have trouble getting the engine to run at first, but once you get over that hurdle you should find it surprisingly easy to fiddle with. Croteam were apparently very serious (heh...) when they said that the engine is well commented and easy to modify.

If game development is one of your hobbies you should probably give it a try, who knows what interesting knowledge can be gleamed from an engine that's been built up through years of use.