Castles & Crusades is a rules light fantasy Role Playing Game that uses the OGL. It plays very much like Dungeons & Dragons. If you want to know what version of D&D it plays like, just ask yourself what version you play.
It is the Rosetta Stone of RPGs. In it you find a simplicity of form that allows you to do anything you choose, with the added benefit of being highly adaptable, which in turn allows you to place rules upon the rules. You can add your own complexity.
If you want to get started, you can scoop up the Starter Pack right here and wade into battle!