There are many pitfalls and traps involved in designing and implementing multiplayer computer games. These pitfalls can often be amplified when legacy single-player games are converted to multi-player. This lecture provides helpful hints and practical examples for programmers and designers of multiplayer games, drawing from our experience in designing network games based on SimCity. Specific attention is paid to issues raised by bringing single-player legacy games, such as SimCity and Civilization, into the multiplayer world.