Civilization V supports extending the base game via mods and scenarios. Mods extend the game by adding or changing anything from civilizations to units. Scenarios are pre-built worlds which place the player into a certain situation, sometimes with special conditions for victory.
Civilization V and its DLC come with a set of scenarios, and additional mods and scenarios can be downloaded via Steam Workshop.
Civilization V doesn't ordinarily support mods or scenarios for multiplayer games. However, Giant Multiplayer Robot allows you to turn a single player mod or scenario game into a multiplayer hotseat game.
The basic idea is that a mod or scenario game is created in Civilization V and then submitted while it's still the game host's turn. GMR then sends the save file back to the host with the game type changed to hotseat and the other players added in.
In the My Games page, click the button that says Create a new game. This will display a dialog with a few game options. From here, simply select the type of game that you'd like to create.
The Basic Mod type is for modded games which add to the base game, but do not replace any of the base game civilizations.
The Total Mod type is for modded games which replace base game civilizations. Selecting this means that all players in your game will show as "Unknown" on GMR.The Scenario type is for games that have already been setup, which includes actual scenarios or existing games. For the Scenario type, you'll need to upload the initial save file before creating the game. GMR will then setup the game for you with the appropriate players and civilizations.
Once you've created your game, be sure to communicate which mods it uses. The game comments are a good way to do this. All players in your game will need to install the mods you're using.
Before creating your mod game in Civ V, make sure to disable any DLC you aren't using – this is especially important if you're creating a vanilla Civ V game and have Gods and Kings installed. To do this, select the DLC menu option in the main menu. Then, select Mods. Upon entering this menu, Civ V will update or download any pending DLC you have subscribed to in Steam Workshop. Once this is complete, select the mods you wish to use and click Next.
From here, the mod setup can vary depending on the mod. Most of the time, you'll select Single Player Game and then Set Up Game. Sometimes, especially for scenarios or custom maps, you may need to select Custom Game or use special settings in your game setup. Check with the mod's page on Steam Workshop for instructions on using a specific mod.
Alternatively, for official scenarios, you can select Single Player from the main menu then select Scenario.
For all mod and scenario games, you'll need to give GMR the save file to turn it into a hotseat game and setup the other human players. For Basic and Total Mods, this will occur after you start the game. For Scenarios, this will occur when you create the game.
For the first turn, you'll need to save the game and submit it before clicking next turn. You'll find instructions on where to find your save file on the Game Details page. After submitting this turn, it will still be your turn. GMR will give you back the save file, and you'll need to load it from the hotseat menu and play it normally.
From this point on, the game will play like all other GMR games.
If you run into problems with your game crashing, try installing and using the Custom Advanced Setup Screen mod to load your hotseat save files. To do this, first subscribe to the mod in Steam Workshop. Then, select Mods from the Civilization V main menu and enable all the mods your game uses plus Custom Advanced Setup Screen. Then, select Single Player -> Custom Game -> Load Hotseat Game. From there, you can load your modded hotseat save file and play your turn.
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