VVVVVV (the letter ‘v’ six times) is about reversing gravity. Like many of developer Terry Cavanagh’s games, it has a minimalist look and is very, very tough!
Start playing from the beginning, and notice how beautifully the game gradually builds on its simple mechanic of switching gravity.
In January 2020, for its 20th anniversary, VVVVVV went open source! Meaning you can now see exactly how it was built! This blog post announcing the decision also contains notes on the code.