SNK's slowly been releasing all of its great old Metal Slug games from the Neo Geo library, and just recently, it worked its way up to Metal Slug X, which I think is the fourth iteration of the series? It's hard to keep track -- there's like twenty of them out there in history.
At any rate, if you've ever played a Metal Slug game before, you know what to expect here: You control a soldier that runs on a constantly moving 2D battlefield, firing away at enemies and dodging all sorts of colorful incoming fire. The Metal Slug games are known for their high quality 2D animation, and this game is no