"Labyrinth" by Gerald Werner (1981) 

This is a port from Coco to TRS-80 MC-10 with MCX (SD or 128) Extended Basic. This game must be one of the earliest BASIC games of the "First-person shooter" genre. It is of a similar time frame as the machine language game "Phantom Slayer", which I think probably stole some of its thunder. But this one is also impressive!