When I was young I used to make a lot of levels for Tomb Raider (3? 4?) because I found the process of making sacred-but-spooky feeling places very enchanting. I had no one to share them with, so it was more like just making folk art for myself and my brother. This feeling was not surpassed until Minecraft came out (and then that was not surpassed until I bought land).

The levels are fundamentally comprised of square blocks that you push up from the ground, and maybe tilt one of the four sides to make shapes. To make caves and rooms, there is a second set of blocks that you pull down from the ceiling!

To get an idea of what this feels like, see this particular tutorial video of theirs: https://tombengine.com/docs/extended-geometry-update-1-7/

The homepage says "The open-source engine" but I can't find a link to the source code anywhere on the site. The link that says "Download" is a link to some Windows binary.

Edit: https://github.com/TombEngine/TombEngine that seems to be the code.

This is more interesting: https://github.com/MontyTRC89/Tomb-Editor (the level editor that goes with the engine)

What made the eternal life of Quake has been its open source level editor which means designers can have a field day making new levels.

The editor is as important as the engine, if not more. Exciting stuff!

Another very cool project in this space is TRX, which is based on a decompilation of the original Tomb Raider 1 & 2 source code:

https://github.com/LostArtefacts/TRX

Of the various Tomb Raider fan engine projects, this is basically the "vanilla, but modernized and well-maintained" historical archival option.

I contributed the MacOS port to this one, and last year I dabbled a little in porting an older random git snapshot to the web:

https://eikehein.com/stuff/sabatu/ (using a fan level, as to not post the original game content online)

It's a very rough hack, so it's not upstream at the moment.

The TRX team is a wonderful and passionate bunch of people to hang around.

From the footer:

> "TombEngine is not be sold."

Ironically, perhaps, this makes me trust it more.