The player must provide a "Doom" source port such as "PrBoom", "ZDoom", "Boom", "GZDoom", or any other Boom-compatible source port.
The player must provide a "Doom" source port, which are unofficial ports of the Doom engine. The webisite recommends using "PrBoom-Plus" and/or "Odamex" or any other Boom-compatible source port.