[In an in-depth design article originally published in Game Developer magazine, Street Fighter II HD Remix designer Sirlin discusses who's done things right -- and wrong -- when creating save systems for video games.]
I once heard Peter Molyneux say that during the development of Populous he didn't want the player to be able to pause the game. His reasoning was that Populous is a world that goes on with or without the player. Luckily, his friends talked him out of it, pointing out that sometimes the doorbell rings, the phone rings, or the baby cries.
Players should be able to save anytime they want, or more precisely, they should be able to stop playing your game anytime without losing their meaningful progress.
This is an old argument where one side talks about the convenience of saving anytime and the other talks about the need to make games challenging, but this is a false dichotomy. We can allow the player to stop playing without excessive penalty and make a challenging game. It's just a matter of defining what "saving" actually means.
As an example, Mario 64 doesn't literally allow the player to save anywhere they want, but it still meets this requirement in spirit. The point of the game is to collect all 120 stars, and every time you collect a star, you "save and continue." You cannot save your exact position in a level, but such a feature isn't needed anyway.
The geography of the game is designed such that a player can reach the entrance to any level in just a few seconds by navigating Mario's castle and getting back to any specific goal in a level doesn't take long either.
This preserves the game's difficulty (players can't save and load to get the stars more easily) and it also means the player can turn the game off at any time, knowing that the only important progress (collecting stars) has been saved.
God of War 1 and 2 and Resident Evil 4 all use the same save system, which is also common in many other games. They all have save points and check points. Save points let players save their progress and load it later. Check points are sprinkled invisibly between save points and if they die, they go back to the last checkpoint rather than all the way back to the last save point.
This system isn't too bad, but it doesn't do a good job of letting the player save and quit at any time, either. It would make more sense if the player could pause the game at any time and save progress up to the last checkpoint.
I'm not suggesting that the player should be able to take a step, save, fire a shot, save-just that he or she should be able to stop playing the game and resume from the last checkpoint. After all, that would happen anyway through dying.
Why separate save points from check points in the first place? I think the answer is for technical reasons rather than design reasons. God of War was designed for the PlayStation 2 and Resident Evil 4 originally appeared on the GameCube (and later on PlayStation 2 and Wii).
These consoles take a few seconds to write a save to the memory card, so doing this every time the designers wanted a checkpoint would probably have been too annoying to the player. This lead to spread out save points and the addition of check points for convenience's sake. In the future, we won't have these technical restrictions.
Gears of War was designed for the Xbox 360, a system capable of writing a save file quickly. Gears of War's save system is a definite improvement over God of War's and Resident Evil's: The player can play through the entire game without having worry about finding save points, but can also quit playing at any time and automatically start at the most recent checkpoint.
Gears of War does this by having many checkpoints, all of which automatically save progress without any action required from the player.
This example well-illustrates the false dichotomy I mentioned earlier. The save system is both very convenient and does not interfere with the difficulty of the game. In fact, Gears of War could be tuned to be arbitrarily difficult without sacrificing any convenience in its save system.