1991 was a incredible year for side-scrolling beat'em up (or hack n slash) games. My three favorite games of all time from the genre, came out in that year. Two of them, are hidden gems that most people probably never played. And one of them is Knights of the Round. The game gives you three characters: Arthur, Lancelot and Percival, in a medieval setting, you just go right and kill lots of enemies with your sword or axe. It had a really nice way of upgrading your armor/weapon each time you hit a certain amount of gold (that you got from collecting itens through the levels).
The game looked and felt amazing to play, being a really good couch co-op as well. One that everybody should give it a shot.