Defense of the Citadel (DOTC) was built on a custom in-house game engine called R5, with the intent to dogfood and stress-test the engine's development. Despite the game never making it to release — and the engine eventually being phased out in favour of Unity — the project was one of the most technically formative experiences of my career.
I had the opportunity not only to design and build a game and all its required systems, but to construct core engine systems from scratch: animation, lighting, shadows, and input. Working at that level of the stack gave me an understanding of game engines that has informed every project since.