...The game contains 'assets' subdirectory with some stages, but you can also design your own stages and the shapes of monsters or tokens in a web-browser with the 3D editor at https://mki1967.github.io/mki3d/ .
Run the game with the path to assets directory as the command line argument.
The assets directory contains the following sub-directories:
'monsters' - monster shapes '.mki3d' files
'stages' - designs of stages '.mki3d' files
'tokens' - token shapes '.mki3d' files
Just place the files with your designs in the respective sub-directories of the main assets directory. ...