Anyone ever used this? Is it purely a c API, or is there also a GUI?
I would say that this is not a tool for everyone, but everone who appreciates time-proven UNIX principles & tooling could find a hidden gem here.
Does it have an assembly module?
From what I get from it, it's more similar contemporary (80ies) CAD systems. Once found a manual for EUCLID in my fathers engineering time-stuff and it looked similar.