What is "mapster"

Mapster is an enhanced version of the Build Editor, the map editor for computer games using the Build Engine, including Duke Nukem 3D and Shadow Warrior. Authored by Richard Gobeille (TerminX), it was built on Ken Silverman's Build source release and re-added most of the features of Duke Build as well as some significant new ones.

Later TerminX also produced Mapster32 based on duke build and maintained as part of the EDuke32 codebase.