GameZebo: What do a metal slug and a regular slug have in common? Neither should come in contact with salt. Seriously though, the Metal Slug series has long been regarded as some of the best run-and-gun offerings the industry has to offer; in the ‘90s, some genre enthusiasts even dropped more than $600 on a Neo-Geo console to play the arcade classics at home. Luckily, you no longer have to give up a kidney to get access to the Metal Slug series. In fact, Metal Slug Wars, a real-time strategy (RTS) game for iOS, will be free to play.