In Gal civ 2 you can set the number of turns it takes before it autosaves already.
I did not know that. I immediatelly set this option in my GC2. The problem is not in Alt-Tab. If you know the application crashes, you would not do this or at least you would save the game before the attempt manually.
However loosing focus is unpredictalbe. An antivirus program reports it is being updated, a battery is low, an e-mail is recieved.....
After losing the focus the game data should be intact. The game runs in the background but it is unable to restore the graphics back to the working state. However you may be right and it might not be the right time as I suggested.
How about such scenario:
1) begin of the turn - the silent autosave is done. If there is old one, it is overwritten
2) if it is the end of turn, start another turn (= go to 1)
3) if the application is being closed by user, erase this silent autosave. The game is being terminated, but it is OK.
4) if the application starts and this autosave exists, the game crashed or something similar happened => user will likely want to have the last working state. In that case ask the user what to do:
4a) user wants to reenter the game => load the game
4b) use does not want to reenter the game => erase the autosave and start the game from the beginning
Such scenario has one great advantage, especially in the beta. If the player wants to send a bug report, he will be able to send very accurate data.
Note: Normal autosave (like those in GC2) should remain too. This special autosave should not be in the list of saves or at least it should have some special name, so user can it easily distinguish the difference. It may have a name like "the last turn" or something like this.