A BSG file contains a machine created by Spiderling Studios Besiege, a medieval game where players build machines to destroy castles. It stores information about a machine, typically a vehicle, in XML format.

BSG files can be loaded in Besiege to play the creations in the game. The files can be uploaded to BSG File Editor, which enables you to modify machines and vehicles stored in BSG files. You can also view and modify BSG files in a simple XML editor or text editor. BSG files may be shared among Besiege users to load customized machines. BSG files can be loaded in Besiege by selecting "Load machine" in Besiege or placing them in the following locations: Windows - \Besiege_Data\SavedMachine of the Besiege installation directory. macOS - Right-click Besiege in "Applications" folder, then place the file in the "SavedMachines" folder, which is located in the "Contents" folder. Linux - \Besiege_Data\SavedMachine of the Besiege installation directory.