It was C# with ASP.NET containers. Not the sexiest tech stack in existence, but it's really versatile. I kind of simplified the architecture in my post, there were like almost a dozen different 'roles', and not all of them ran off the same executable (since some roles had specialist requirements, with some running off of Windows, the others off Linux, so there was some real world messiness to account for), but the Standalone 'god' mode was there and pretty much worked well on dev machines.