The tools that were used to make Lemmings were shown in a 2019 GDQ talk, called Classic Game Postmortem: Lemmings [1]. The editor, and how fast the loop was between tweaking a level a little bit and being able to try it again, are pretty impressive. The talk also goes through the wonders of high quality low pixel animation.

[1] https://www.youtube.com/watch?v=ybs5FR-uUNI