Free Hero Mesh

 you are using on your computer.

** Is there documentation with tutorial or introduction?

 I like the kind of documentation that the existing one is, although I
 am aware that some people prefer a different style. Someone who is better
 at writing that different style should do so and contribute please.

** How was the working of the original game engine figured out and then
Free Hero Mesh made to emulate its behaviour?

 This involved several parts.

 The file format was figured out by examining existing files, and making
 files in the original engine with minor changes of each other to find
 which parts of the file were changed.

 The working of the game play was figured out by reading the documentation
 included with the original game engine (which has many omissions and
 mistakes) and by making many hypotheses and scientific experiments and
 examining what behaviour resulted from that, in order to come up with
 theories of its working, testing them, and implementing them.

 Further tests were made by replaying the included solutions in Free Hero
 Mesh and confirming that they result in a WinLevel. (This is one of the
 earliest features implemented in Free Hero Mesh, and helped with testing
 the game engine.)