MMORPG Video Game of the year 1999 created in Visual Basic 6.0 by Pablo Marquez (Morgolock), this game is open source.
I have translated in C++ under SDL2, this game would be your base client, it would be necessary to implement the multiplayer with your server.
Visual Studio 2022 is needed (although with Code::Blocks, DevC++ or any other IDE you can use it, if you know how to build the project).
You must also install and include the SDL2 library along with SDL_Image and SDL_Mixer.
Steps for windows: Lazy Foo tutorials SDL2: Setting up SDL on Windows