There may not be a more complicated genre in video games than the Metroidvania. This portmanteau of Metroid and Castlevania started to gain traction in the early 2000s - thanks to games like Cave Story - as a very particular style of game with a fully connected and explorable map, with rooms and areas that are gated either by missing abilities or weapons. It's an altogether fascinating genre, one that has produced some of the greatest games ever made.