The World Learns to Be Shared
The pieces that turn one map into a world several people can stand in.
A copy of the game now knows what it is — alone, server or client — and the server keeps the accounts and the characters as saved records instead of a file on someone's machine.
It also learned who needs to hear about what. The world has hundreds of creatures and tens of thousands of squares; sending all of it to everyone twenty times a second would spend bandwidth on things nobody can see. You are told about what is near you, and nothing else.