This new way of making caves looks neat. You're using those bordering walls as reference?
Yep, an idTech 4 with idTech 5 rendering code and further modifications. This is just messing with engines in general. And I couldn't get it to work before... just had to prove myself that I can resurrect it. If I can manage to improve game logic and assets further, I could make my project on it.
The engine comes with numerous in-built tools (lights, GUI, ragdoll, particle editors, script debugging). It just has much less documentation than UE4 (I find blueprints and it's other graphical editors more confusing than writing doom3 scripts...), full collision model, AI, area awareness system with dynamic obstacle avoidance and proprietary physics engine (proprietary = written by id, GPL).