Not sure if this helps, but I've found a workaround for Sierra. If you open the app's resources folder (/[path to folder containing game]/the\ Peter\ Navarre\ Crecy\ Evaluation.app/Contents/Resources/) and copy everything in that folder to the app's support folder in your user library (/Users/[your username]/Application\ Support/com.mateuszskutnik.crecy/) and then run the game via the Terminal app (Utilities folder) using the command "/[path to folder containing game]/the\ Peter\ Navarre\ Crecy\ Evaluation.app/Contents/MacOS/Mac_Runner", it works perfectly, minus some temporary glitching with full screen.
Great game by the way! Well done as always!