Dark:
When you lock the screen, you'll get a game centre notification when someone plays. All the notifications you got while the screen was locked will show up on the lock screen, the screen that you get before you slide to unlock. You can also go to the bars at the top of the screen (where battery life etc is shown) and pull down with three fingers to see your notifications. You can feel free to start another online game. When your screen is locked, you will get notifications for all the games you're playing. When you're in lost cities, it will show you a list of all the games you're playing, and you can toggle between them at will.