Hold the Fort
The invasion was a surprise and no one was prepared for it. A huge part of the human race was erase, but there was a quite few, who manage to survive. You manage to find an old fort with some human survivors and convinced them to fight. As the title suggest, you need to hold the fort. You need to shoot the invading monsters during the night. During the day, you can repair or do anything that you want, but don't forget a rest. As you make some progress, others would join your cause. The game would be harder as you make some progress.