Back in 2013, I wrote a post about Spelunky HD and its demonstration of "rule horizons," a concept I described as "expanding layers of complexity that radiate from a central core ruleset." Like many games that strike a delicate balance between simplicity and depth, Spelunky expands with the player's expertise. For the casual player, it's a cartoonish platformer with destructible terrain, obtuse mysteries, and a comical ecology of physics interactions. Place a bomb too close to a lava pool ...