It's interesting that one of the first things he does is to effectively build a REPL - the executable is a shim and the actual game code is kept in a shared library which is recompiled and reloaded on the fly. Together with manually laying out memory instead of relying on the runtime malloc/free mechanism for allocations, it seems to work pretty well. Make a code change, compile, the game updates immediately. Thanks to sticking to C the compile times are short enough to make the reload cycle near instantaneous as well.