For Ion Maiden, we're using BUILD. The old-school engine which powered games like Duke Nukem 3D, Shadow Warrior and Blood back in the mid '90s.
Working with legacy technology has been quite a challenge. If you're curious, we just released a new developer blog called "Teaching an old engine some new tricks", where we dive into some of the challenges and solutions we came up with, while working on Ion Maiden. Check it out!
https://3drealms.com/devblog/dev-blog-3-teaching-old-engine-some-new-tricks/