I started a sequel to Crystal Towers in 2007, not really intending it to grow as big as it eventually did. The simple platform game mechanics of the original were transposed into a hub-based world, with multiple routes and bosses, Zelda-style ability gathering and even a synthesis side quest being added along the way.
It took four years to finish, which it shouldn't really have done if I'd planned it a bit better - nevertheless, thanks in no small part to the efforts and enthusiasm of the multiple friends I got to go playtest it, it was finally released in June 2011, to my lasting relief.
It's easily my biggest game ever, with the full edition taking people about 40 hours to play through to 100%. Both that and a cut-down free edition are available on the game's site.