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Any multiplay game can be rough to develop unless you have a skilled software engineer on your team and a good engine to back it. Even Unity and Unreal have a hard time making multi player games easy for people to build. A work around is Co Op or split screen. 3 Types of Multi player games: Local (Co Op), LAN and WAN. Local is the easiest to setup but limited. Typically seen on console games like X Box. LAN is pretty common years ago for LAN parties. But can still be useful if you use tools like Hamchi. WAN is the most difficult and expensive to invest in. WAN requires a Server  & Client setup. You can make a Server + Client application. However, a true MMO has a dedicated Server and clients connecting to the Server. I hope this helps explain the different types and helps you better structure your project.