Creator's site: http://www.remar.se/daniel/herocore.php
RPS article: http://www.rockpapershotgun.com/2010...
The default difficulty is worth about 2 hours of playtime, but there's nothing stopping you from "finishing" the game relatively quickly, provided you can find your way to the final boss. The higher difficulties apparently change the world map completely, also significantly adjusting the bosses.
It's really worth checking out.