Over Horizon is a side-scrolling shoot'em up video game developed by Hot B for the Nintendo Entertainment System, and published by Hot B in 1991.
Over Horizon comprises six stages, each of which concludes with a boss battle. Players can also edit weapons to act differently than their usual selves. In a way, it helped to turn the average gamer into a budding game designer.
Source: Wikipedia, "Over Horizon", available under the CC-BY-SA License.