After solving the Indians puzzle in level 12 of vim-adventures, one directly goes to level 14. I am curious that where is the level 13 in the game.
There is no level 13.
The level 13 is in fact missing. That's because the number 13 is a bad luck number. Many times (especially in the US) people consciously avoid the number 13, because it is believed that it causes bad luck. This is the developer's choice and has nothing to do with the game itself. The game has 13 levels numbered from 1-14, excluding the level 13.
Officially this kind of behavior is called triskaidekaphobia.